Journey's End (TV story)

"For this is my ultimate victory, Doctor! The destruction of reality itself!!"

- Davros

This is the 13th and final episode of Series 4 and featured 7 companions of the Doctor. It is a continued on a cliffhanger from Episode 12.

Synopsis
The entire universe is in danger as the Daleks  activate their master plan, and enslave 21st century Earth. The Doctor is helpless, and even the  TARDIS  faces destruction. The only hope lies with the Doctor's secret army of companions– but as they join forces to battle Davros himself, the prophecy declares that  one of them will die.

Plot
Following on immediately from the end of "The Stolen Earth", The Doctor is regenerating inside the TARDIS while Donna Noble, Captain Jack Harkness and Rose Tyler watch in horror. However, the Doctor transfers his regenerative energy into the container which carries his severed hand. He has healed himself, but chosen not to change his appearance. The TARDIS is transported by the Daleks to the Crucible and rendered powerless. The Doctor, Jack, and Rose leave it, but Donna is distracted because she is hearing the sound of a heartbeat and while looking back, the TARDIS door slams closed. Before the Doctor can free her, the Daleks dump the TARDIS into a waste chute where it will be destroyed in the centre-core of the Crucible. As the TARDIS interior explodes around her, Donna collapses near the severed hand, she hears the heartbeat again and while touching the container energy flows between it and her. The hand bursts out of the container, and forms as a new Doctor, although this Doctor has only one heart and has picked up some of Donna's mannerisms. With his help, the TARDIS escapes destruction and gives the new Doctor and Donna time to come with a plan.

In Torchwood Three, Gwen Cooper and Ianto Jones find themselves safely in a time lock created by Toshiko Sato, preventing the Dalek from entering but also preventing them leaving. Sarah Jane Smith is saved from two Daleks by Mickey Smith and Jackie Tyler, but in order to follow the Doctor, lay down their guns and allow themselves to be captured, taken to the Crucible. Martha Jones says her goodbyes to her mother and makes for an abandoned castle in Germany where one of five Osterhagen stations is hidden, and waits for contact from the other bases.

Aboard the Crucible, Jack creates a distraction by shooting the Supreme Dalek (Red Dalek) with his revolver, but is shot by the Daleks; as the Doctor and Rose are taken to the Vault where Davros is held, Jack's immortality allows him to escape. With the Doctor and Rose contained, Davros explains that the 27 planets form an energy pattern that is then amplified into a "reality bomb", able to break apart the forces holding everything together. Mickey, Jackie, and Sarah Jane escape a test chamber where this effect is shown to the Doctor just in time. Jack finds his way to the three, and with a locket from Sarah Jane, creates a device that will implode the Crucible. Meanwhile, Martha makes contact with two other bases in China and Liberia. The Chinese counterpart wants to get it over and done with, but Martha, knowing the Doctor, first broadcasts a signal to the Crucible to give them (probably both Earth and the Daleks) a second chance, promising to use the Ostenhagen key to detonate 25 nuclear warheads under the Earth's crust to destroy it and disable the reality bomb. However, the Daleks manage to lock onto their positions and beam Martha, Jack, Mickey, Jackie, and Sarah Jane, with the Transmat to the Vault where the Doctor and Rose are too being held captive. The Daleks prepare to activate the reality bomb that will wipe out all matter in this and every parallel universe through the rifts in the Medusa Cascade, but the new Doctor and Donna arrive in the TARDIS. Both, however, are stunned by shots from Davros. The reality bomb countdown reaches zero, but nothing happens; Donna has manipulated the controls to disable it. The Doctor recognises that the creation of the new Doctor has had an unintended side effect: Donna is now half Time Lord herself, sharing the Doctor's intellect. Donna and the new Doctor free the others, and with the help of the original Doctor, disable the Daleks and start to send the planets back to their proper time and space. Before Earth can be sent, the machinery is destroyed by the Supreme Dalek, who is then destroyed by Captain Jack. The original Doctor races into the TARDIS to replace the functionality of the broken machine. Realising that Dalek Caan has seen the end of the Dalek race and has been manipulating time to achieve this, the new Doctor (probably not kept back by guilt due to the influence of Donna's personality) uses the remaining machinery to destroy all of the Daleks and their fleet. The rest of the companions flee to the TARDIS, and while the Doctor offers to save Davros, but he refuses, calling the Doctor the "Destroyer of Worlds". The Crucible is destroyed.

The Doctor enlists the help of the other companions, making contact with the base Torchwood and with Luke Smith, Mr. Smith and K-9, to help use the TARDIS return the Earth to its proper place. Sarah Jane says her goodbyes, as well as Jack, Martha, and Mickey, who has decided to stay in this universe. Using a retroactively closing rift, the Doctor returns Rose and Jackie to the alternate dimension and leaves the new Doctor with her, as he will now grow old with Rose, no longer able to regenerate due to the human influence, the human doctor, having the same memories and feeling as the proper Doctor, tells Rose he loves her and they kiss.

Returning to their universe, Donna finds she begins to have trouble thinking; the Doctor explains that the human mind cannot take in the Time Lord mental abilities. To save her, he wipes her mind of all her encounters with the Doctor, returning her home and explaining to her family, Sylvia Noble and Wilfred Mott, that she must never be reminded of her time with the Doctor or else she will die. As Donna recovers consciousness, she shows no interest in the Doctor; he leaves, though Wilfred promises he will look out for the Doctor every night while he looks at the sky. The Doctor then returns to the TARDIS, alone once again. Waiting for his next adventure........

Cast

 * The Doctor - David Tennant
 * Donna Noble - Catherine Tate
 * Rose Tyler - Billie Piper
 * Martha Jones - Freema Agyeman
 * Captain Jack Harkness - John Barrowman
 * Sarah Jane Smith - Elisabeth Sladen
 * The Doctor - David Tennant
 * Mickey Smith - Noel Clarke
 * Jackie Tyler - Camille Coduri
 * Luke Smith - Thomas Knight
 * Gwen Cooper - Eve Myles
 * Ianto Jones - Gareth David-Lloyd
 * Wilfred Mott - Bernard Cribbins
 * Sylvia Noble - Jacqueline King
 * Francine Jones - Adjoa Andoh
 * Davros - Julian Bleach
 * Dalek Voice - Nicholas Briggs
 * Voice of Mr Smith - Alexander Armstrong
 * Voice of K-9 - John Leeson
 * German Woman - Valda Aviks
 * Scared Woman - Shobu Kapoor
 * Anna Zhou - Elizabeth Tan
 * Liberian Man - Michael Price

Production crew

 * Dalek Operators - Barnaby Edwards, Nicholas Pegg, David Hankinson, Anthony Spargo, Gethin Jones

Individuals

 * Those shown in flashback who died for the Doctor are Harriet Jones, Jabe, The Controller, Lynda Moss, Robert MacLeish, Mrs Moore, Colin Skinner, Bridget Sinclair, Ursula Blake, Face of Boe, Chantho, Astrid Peth, Luke Rattigan, Jenny (who is in fact not dead, but the Doctor is unaware of this), River Song and the Hostess.
 * Both Rose and the Doctor recognise the familiar resemblance between Gwen Cooper and Gwyneth (who they encountered in Cardiff in 1869).
 * Rose and Mickey, who previously had an on again, off again relationship, appear to have drifted apart. They do not look at each other, speak to each other, or interact at all, even when they are in the TARDIS together. Mickey does not say goodbye to her (though he does say goodbye to Jackie saying he'll miss her "more than anyone") and he tells the Doctor there's nothing for him in the parallel world, "certainly not Rose".
 * Just before the Doctor is forced to erase her memory, Donna expresses a desire to meet Charlie Chaplin. This is the second finale in a row to have a character state a desire to meet a famous 20th century personality; previously the Doctor told Martha he wanted to meet Agatha Christie (DW: Last of the Time Lords); Christie subsequently appeared in The Unicorn and the Wasp; it remains to be seen if Donna's reference also serves a foreshadowing.

TARDISes

 * This is the first episode where the TARDIS is fully-staffed with six pilots, and the first time it is noted definitively that it was designed for six, after various mentions about it being made for more than a single Time Lord.

Technology

 * The purpose of the Osterhagen key is revealed in this episode.  Martha's key is one of several required to set off a network of nuclear weapons buried deep beneath the Earth's surface.  If detonated, these weapons would trigger the explosion of the Earth.  Each key must be inserted into a control panel at an "Osterhagen station". There are apparently five around the world, but only three need to be manned with a key to initiate the detonation.  Locations seen on screen are Germany, Liberia and China.  The "Oserhagen Project"  appears to have been in place for decades, according to the German woman who supplied food to the guards at the German station.  Given the age of the German woman, and her claim that she knew of the Osterhagen key when she was in London during her youth, the "Osterhagen Project" likely dates to the days when the Brigadier was in charge of the British arm of UNIT.
 * The technology used to emplace the nuclear weapons at the Earth's crust could therefore be linked to the drilling project featured in DW: Inferno''.


 * The Daleks have access to transmat technology.
 * The TARDIS is captured by the Daleks in what they call a temporal prison but what the Doctor calls a chronon loop.
 * Toshiko Sato installed a time lock around The Hub at Torchwood Three.
 * Sarah Jane uses her sonic lipstick in the same manner as the Doctor's sonic screwdriver. This is the tool's first appearance in the main series.
 * The "three Doctors" send the planets back to their original position through the use of a "magnetron". It is unclear if this is an intentional reference to the magnetrons seen previously in the original series (DW: Day of the Daleks, The Mysterious Planet), or just a general reference to real life magnetrons, used to power microwaves, radar screens and televisions.

Story notes

 * Blue Peter presenter Gethin Jones operates a Dalek in this episode, returning to Doctor Who since his brief appearance as a Cybus Cyberman in The Age of Steel.
 * This was the longest series finale at 65 minutes long, longer even than most of the Christmas specials, except for Voyage of the Damned, which was 71 minutes.
 * Dalek Caan refers to the Doctor as a 'threefold man'. The meaning becomes clear in this episode with both the copy of the Doctor and 'Doctor-Donna'.
 * This episode marks the first series finale to show a preview of the upcoming Christmas Special (2008). After the credits the Cybermen are said to return in the episode. However the episode is unique for being the only series finale in the Russell T Davies era which doesn't end on a cliffhanger.
 * Graeme Harper's penchant for including a distorted image of a main character is present in this story.  Though not included in  every single story he's directed for BBC Wales, it's seen often enough to be considered something of a directorial "signature".  Similar distortion is achieved through the use of  magnifying glasses in Army of Ghosts, The Unicorn and the Wasp, and Utopia,  and with mirrors in Turn Left.   This time, it's Mickey, Jackie and Sarah Jane that get "the Harper treatment" under a curved window.
 * This episode tells us that Harriet Jones, former Prime Minister, actually died in the previous episode.
 * Davros named the Doctor 'The Destroyer of Worlds' and maybe a reference to Fires of Pompeii when it was said the Doctors name was sealed in the Cascade of Medusa herself or to the Doctor being the Ka Faraq Gatri.
 * The Osterhagen key would destroy the Earth. The word, Osterhagen, is an anagram of the phrase, Earth's gone.
 * This story augments the notion that Time Lords have some measure of control over the regenerative process. as seen in Last of the Time Lords.  In truth, most regenerations have added at least a little to the general mythos about the process.  From the notion that a particular physiognomy could be imposed upon the Second Doctor in The War Games, details have been added about how the process works almost every time one has been depicted.  In this case, writer Russell T Davies builds upon his earlier idea that a Time Lord can re-grow whole body parts during "the first 15 hours" following a regeneration (The Christmas Invasion)   Here he suggests that a Time Lord can stop the process prior to entering the final stage, provided that he has a matching genetic receptacle into which he can store the energy.
 * When the newly created Doctor discovers he's "part Time Lord, part human" he is shocked and refuses to admit it. This is likely a reference to the 1996 movie and fan outrage at it. It might also suggest that the Doctor was never half-human due "Human-Time Lord metacrisis"
 * The scene where the Daleks are speaking German is possibly a tongue-in-cheek reference to the fact that Terry Nation based the Daleks on the Nazis.
 * This marks the likely permanent departure of Catherine Tate (Donna Noble) and Billie Piper (Rose Tyler).
 * The story elements surrounding the destruction of the universe have some casual similarity to ideas found in Life, the Universe and Everything, a Hitch-Hiker's Guide to the Universe sequel penned by former Doctor Who script editor, Douglas Adams. Everything was in turn based on an abandoned Fourth Doctor television serial had written, called Doctor Who and the Krikkitmen.
 * The recap of the previous episode uses different footage of Jack stating "you know what happens next" in the leadup to the regeneration. In The Stolen Earth he utters the line off-camera, but in the recap he is seen saying it.
 * The Doctor and Mickey perform a "fist bump" in lieu of a handshake when Mickey departs. This mirrors the way they greeted each other in Doomsday.

Ratings
to be added

Myths and rumours

 * The week between the cliffhanger ending of The Stolen Earth and the broadcast of Journey's End included some of the most intense fan speculation and media attention in franchise history. The significance of the cliffhanger, which appeared to show the Doctor regenerating, along with previously reported speculation regarding Donna and other characters led to many speculations being circulated on fan discussion boards and the media. Among some of the most notable:
 * That David Tennant was in fact leaving the series, and that leaked photos and other information regarding him being in the 2008 Christmas special (as well as media reports the preceding week that he was negotiating to return in 2010) were either a "red herring" or that the Christmas special was to include a flashback.
 * The true nature of Donna was the subject of much speculation, with some fans suggesting her to actually be The Rani or Romana living under the influence of a Chameleon Arch, or a manifestation of the Master.
 * Concerning Donna's ring, at the end of the season 4 finale, when the Doctor says good-bye to her it glimmers briefly into the camera. Some fans theorise that the ring is a possible Chamelon Arch containing Donna's memories of her time with the Doctor. It has also been suggested that the ring resembles a ring worn by The Master in a previous episode. Others theorise that the ring is just large, black, and very shiny.
 * And the prediction that a companion would die led some to believe Donna, Martha or Rose would be the ones destined to die (since it had already been reported that John Barrowman would be returning to Torchwood and Elisabeth Sladen to The Sarah Jane Adventures, ruling out their characters' demise.)


 * The appearance of K-9 was a surprise to many as it had been previously reported that the character would not be appearing in the episode, given the fact the rights to the character are currently held by another party for the planned K-9 television series.
 * After Eve Myles, who had played Gwyneth in The Unquiet Dead was cast as Gwen Cooper in Torchwood, Russell T Davies stated in an interview in Doctor Who Magazine that the characters were unrelated. In this episode, however, Davies reversed this opinion by inserting dialogue strongly implying the two shared common ancestry.
 * There is a possibility that either The Doctor or his twin left behind on Pete's World will eventually manfiest into The Valeyard due to the escalation of pain and abandoment felt each by the other for different reasons

Discontinuity, Plot Holes, Errors

 * Harriet Jones' death means that the "three successive terms" as Prime Minister which the Ninth Doctor remembers in World War Three are now no longer able to happen. Doesn't this create a paradox?  Not really.  As explained in The Fires of Pompeii, some historical events are "in flux" and can be changed; others are "fixed" and must not be altered.  The exact dimensions of her term of office seems to have been a part of history that was in flux. After all, the Ninth Doctor really had to struggle to remember who Harriet Jones was.  If Agatha Christie's life could have been ended following the publication of The Murder of Roger Akroyd, or Shakespeare's after the first performance of Love's Labour's Lost'', a politician's loss of additional terms of office would seem possible.  However, her absence from the political scene has caused the Doctor problems he wouldn't have had to endure had he left her in power.  Namely, it allowed the Master to win election as Prime Minister. Another way of explaining this is because the Master had a paradox engine. Harriet Jones would have kept her terms in office but the Master used the machine. Also, it has been mentioned that Harriet Jones is still Prime Minister in Pete's World, and did we ever get an explination of why the ninth doctor called Mickey, Rickey?
 * If the TARDIS's power has gone, how does the monitor screen work? Strictly speaking, its power wasn't "gone"; the TARDIS was merely in a temporal prison. While this shut down most power, it clearly didn't cut everything, as happened when the TARDIS appeared to "die" in Rise of the Cybermen''.   There was still some power being supplied to parts of the ship not directly used in time travel, such as the running lights and scanner.
 * When the Doctor wiped Donna's mind, it looked like he only wiped the memories of her time with the Doctor. Couldn't he have just wiped the Time Lord stuff that was overloading her brain? However, if she remembers the 'Time Lord stuff' she will burn up, and it would be easy to remember if you're traveling around with one.
 * Wouldn't the nuclear warheads placed under the crust have melted? UNIT would most likely have thought about this, and provided some sort of way to protect them.
 * If Donna is dying because of the "Human-Time Lord metacrisis", why isn't the "second Doctor"? ''Both parties' lives are in danger, but in different ways.  Donna's life is under immediate threat for a specific reason:  her human brain is being burned up by attempting to cohabitate with a Time Lord consciousness.  She's literally trying to put too much "stuff" in her comparatively smaller brain.  It's like what happens when one completely fills up the main disk drive on a computer.  Eventually, the operating system runs out of space and the computer begins to run very slowly.  The only solution is to delete files and free up some disk space.  The "part human" Doctor is suffering a different kind of crisis.  He has the brain of a Time Lord, but the body of a human.  Thus he's not under any immediate threat of dying, but he has only one heart and cannot regenerate.  Thus, he is a "reduced" version of a Time Lord.  From a Time Lord's perspective, he's literally dying as well.
 * Why did Martha have to travel to Germany to activate the Osterhagen key when the other soldiers were in pods in their home countries? The main pod was in Germany - Martha states that she is in Osterhagen 1. There are 5 Osterhagen stations so presumably there will be one in each continent, Africa, Asia, Europe, North America and South America, therefore, Germany would be the site of the European station, while the other two stations being used would have been African and Asian.
 * The moon remained in position when the Earth had moved. The moon should have locked onto the strongest gravitational force (the Sun) and been pulled towards it. There is no indication one way or the other as to the moon's position. The camera angles used do not show anything of  the rest of the Solar System, so it's exceedingly difficult to surmise anything of the moon's relative position.  Any changes to the moon's orbit — provided that the Earth was gone long enough for the moon's orbit around the Sun to decay — could have easily been corrected by two sources.  The TARDIS appears to shunt towards the moon after replacing the Earth, so the Doctor could have given the moon a little "nudge".  Alternately,  the moon could have been "finessed" back into proper orbit by Mr. Smith, who demonstrated precisely this ability in SJA: The Lost Boy.
 * If the act of temporal shifting back to the Time War showed Dalek Caan the entire history of the Dalek race and led him to conclude the Daleks should be destroyed, why didn't he just let Davros die in the war and then kill himself? Caan was driven insane after saving Davros, therefore he had only seen the whole of time after Davros was already safe. He then began setting the course of events that would lead to the fall of the New Dalek Empire.
 * If any mention of the Doctor or the TARDIS would cause the Time Lord consciousness within Donna to reawaken and burn up her mind, isn't the Doctor taking a tremendous risk by letting Donna see him in the Nobles' house? The Doctor wishes to test the effectiveness of the memory wipe and also determine whether there are any negative effects on her. The real risk to Donna is if she starts to remember when the Doctor is absent and unable to intervene before she suffers a complete neurological collapse. Note he only introduces himself to her as John Smith -- an alias he did not use on screen during their travels -- and it is possible the TARDIS, while visible to the viewer, might be hidden behind the perception filter previously established.
 * Why didn't the Daleks activate an Emergency Temporal Shift to escape the control of Donna, and how did the Dalek Supreme manage to avoid being controlled? The Emergency Temporal Shift may have been an innovation of the Cult of Skaro, the secret of which Caan did not share with Davros or the new Daleks. And the Dalek Supreme was seen under the same loss-of-control effect as the rest of the Daleks, but not to such a high degree. It could also be assumed that along with a limited range of emotions, Davros also imbued the Dalek Supreme with a few other special abilities, which may include all his functions not being looped in to the main control panel in the vault. The emergency temporal shift would be turned off anyway, especially since the Doctor's mind would recognise this method of escape.
 * Why were the controls put in the Vault where Davros could access them and destroy the Daleks as the Doctor-Donna did? The fact is that the nature of the relationship between Davros and the Daleks is never fully described in this episode. We have largely only the Tenth Doctor's assessment that the word "vault" means "dungeon" which throws Davros' status into disrepute.  Clearly, there is some kind of power sharing arrangement, but Davros is able to give orders to both ordinary and Supreme Daleks alike.  And the whole Empire is co-ordinating its efforts to accomplish Davros' mission.  This is perhaps unsurprising as these Daleks, unlike others we have seen in the past, are literally made from Davros' cells, and are not derived from the general Kaled gene pool.  Thus, the Tenth Doctor might well be wrong in his assessment of Davros' relative power.  Indeed,  the simplest explanation is that a vault, by definition, is a secure location.  The question isn't, therefore, really why the controls would be in the vault, but why would Davros be there?  Maybe he, in fact, requested it.  Maybe it was Davros' insurance against the Daleks deviation from his plan. If they stepped out of line, he could have some measure of immediate control over them.  That the Dalek Supreme seemed less affected by the Doctor-Donna's manipulation of the Dalek control panel suggests that he was somewhat independent of it, which acted as his safeguard against Davros.
 * The second humanoid woman aboard the Shadow Proclamation ship/station told Donna there was "something on your back", repeating the prediction of Lucius in The Fires of Pompeii. We know the Time Beetle is gone, but there was no explanation concerning the cryptic phrase by the climax of Journey's End. The albino woman spoke in the past tense saying there "was something on your back", the other temporal aberration she noticed was the time fields converging on Donna, the same phenomenon Rose spoke of, which we learn was a product of Dalek Caan's secret temporal manipulations which he implemented in order to hasten the destruction of the Daleks.
 * If Mickey Smith was allowed to stay on the normal Earth, why did Rose and Jackie have to go back? After all, the Doctor could have brought Pete back as well, and their child. Pete would have probably been reluctant to leave his own universe, as he runs a successful business. Also, the Doctor states that his hybrid clone is a liability after committing mass genocide on board the Crucible, and gives him to Rose. He probably doesn't want to be in the same universe with him. It seems that the Doctor was only able to get to Bad Wolf Bay and stay there for a few minutes, so there was no chance of getting someone else back to the normal Earth. Also, it should be remembered that Pete is not from the normal Earth, so it wouldn't necessarily be fair to force him to change worlds. There is an even simpler answer: in the Doctor's universe, Pete has been dead for 22 years. Bringing him back would raise some difficult questions. Indeed, Rose is also assumed to have perished in Doomsday.
 * If the New Dalek Empire was grown from cells taken from Davros, how would the new Daleks know that Davros shouldn't be trusted and so decide to lock him up? It's not that he wasn't trusted, Davros was just not in control. It was most likely the Daleks took over authority naturally, and decided to keep Davros imprisoned so he would not compromise it. Remember, the Daleks see Davros as inferior because he is not a Dalek.
 * When the Doctor sees Gwen Cooper for the first time, he asks if she came from an old Cardiff family. She replies yes, but this cannot be true as in Something Borrowed, Gwen's parents come from Swansea. Obviously, they could have moved there from Cardiff; the two cities are not far apart and her parents could not possibly has been born in the 19th century anyway.
 * Related to the above, Gwyneth appeared to have no family in The Unquiet Dead so how could Gwen be her descendant? Dialogue never indicates the nature of their relationship; Army of Ghosts featured Martha Jones's cousin who was a lookalike for Martha. Victorian families had many children and Gwen could be descended from any of Gwyneth's relations.
 * With all of her memories since The Runaway Bride erased, wouldn't Donna realise that she has lost about a year and a half of of her life, and shouldn't she think it's her wedding day? The exact nature of the mind wipe is never specified. However, the Doctor seems to be pinning his hopes on the fact that the pre-Doctor Donna missed so many major events that she would dismiss gaps in her knowledge about the world around her as "par for the course".  If the discussion between Madame de Pompadour and the Tenth Doctor is any guide to the Doctor's psychic abilities, however, he may have been able to permanently close some "doors" in her mind, since doors "can be entered from either direction".  Given the fact that Time Lords possess the ability to completely change their own physiology and memories via the Chameleon Arch, it is likely that individual Time Lords might find it a easy matter to alter a Human's relatively simple brain with respect to a small percentage of its total memories.  Such abilities were shown before in The War Games. when Zoe and Jamie were deprived of most of their memories of the Second Doctor by the Time Lords.  Furthermore, it is unclear just how much time has elapsed from Donna's perspective.
 * Why do some Daleks have special 'cogs' instead of suckers on their right arms? The "sucker" may well be the standard right limb for a Dalek, but it has never been the only possible limb. At least as far back as DW: The Daleks' Master Plan, other appendages, like flame-throwers, have been seen.  In the BBC Wales series, alternate limbs have been seen since The Parting of the Ways.  The precise functionality of the "cog" arm is not made clear in this episode, however.
 * Jack last encountered Mickey in Boom Town when Mickey was insecure, a bit inept and somewhat unimpressive. Aside from tongue-in-cheek joking around with him, Jack doesn't display any doubts about Mickey's abilities or any shock that he is involved in fighting the Daleks. Although Mickey is somewhat lacking during the chase sequence in Boom Town, he does display considerable resolve when directly confronting Margaret Blaine later in the episode. Furthermore, as a member of Torchwood, Jack may be aware of Mickey's role in the events of Army of Ghosts and Doomsday, perhaps by viewing internal video camera footage salvaged from the site.  He definitely knows Mickey was in the parallel world with Rose and Jackie from a conversation in Utopia.  He may assume that Mickey can now handle a gun since he first encounters Rose in The Stolen Earth wearing the very gun he tosses to Mickey in this episode.
 * When Jack got Gwen's name wrong — he said her surname was Cooper rather than Williams — why didn't she correct him? There is no definitive indication in Torchwood that Gwen took Rhys' surname after marriage.
 * If the Osterhagen key and its associated doomsday device had been around for years, why wasn't it activated during the events of Doomsday or, more to the point, The Year That Never Was (DW: Last of the Time Lords)? If Harriet Jones knew of the key, The Master, in his Harold Saxon persona, would have known about it and presumably deactivated it before he launched the Toclafane attack.
 * When the Daleks were in Germany, why did they not use the proper translation of "Ausrotten" when shouting "Exterminate"? Perhaps an error in the programming or, possibly, Martha is experiencing some sort of residual translation left over from her time in the TARDIS. Either that or it was done for comedic purposes in an otherwise dark episode.

Continuity

 * When the Doctor sees Gwen Cooper for the first time, he asks if she comes from a long line of family from Cardiff. This is because of the similarity between Gwen and Gwyneth (DW: The Unquiet Dead), both of whom are played by Eve Myles. The Doctor and Rose both recognise the uncanny family resemblance.
 * This is the first occurrence of the Doctor's TARDIS being piloted by six people, that number first being specified in NA: Cat's Cradle: Time's Crucible.
 * This episode marks the last appearance of the Tenth Doctor's severed hand which first appeared in DW: The Christmas Invasion and throughout the first season of Torchwood.
 * Davros mentions meeting Sarah at the birth of his creations; this happened in DW: Genesis of the Daleks.
 * Mickey Smith and Jackie Tyler last appeared in DW: Doomsday.
 * Donna tells the Doctor how to fix the Chameleon Circuit which has been broken since DW: An Unearthly Child. The Sixth Doctor had previously attempted this in DW: Attack of the Cybermen, as had the Fourth Doctor in DW: Logopolis.
 * This is the third time a Doctor has been depicted in a way to suggest he was unclothed. The first time was in Spearhead from Space in which a newly regenerated Third Doctor took a shower. The second was during the regeneration from the Seventh to the Eighth Doctor, where he was merely covered by a sheet. The Ninth Doctor appeared shirtless during the torture scene in Dalek

DVD and Other releases

 * This episode will be released as part of Series 4 Volume 4 DVD alongside Turn Left and The Stolen Earth on 1st September 2008.
 * It will also be released in the Series 4 boxset in November 2008.