Cold Fusion (audio story)

 was the eleventh and final story in Big Finish Productions' Novel Adaptations range. It was adapted from the novel of the same name by Lance Parkin and featured Peter Davison as the Fifth Doctor and Sylvester McCoy as the Seventh Doctor.

Publisher's summary
A newly regenerated Fifth Doctor arrives on an occupied ice planet – where the Seventh Doctor is investigating dangerous energy experiments conducted by the Earth Empire. But events spin out of control when a refugee from the distant past arrives - Patience, the Doctor's Wife!

Part one
The Seventh Doctor travels to a waystation and meets with Ziyou Wanle, a former prospector who recounts how he and his team came across a machine buried in a cavern and a ghost emerged from it before killing Ziyou's teammates. A similar ghost appears and they run, but they are separated after the Doctor is confronted by a wardroid which he manages to shut down and the Doctor leaves by transmat when the Adjudicators arrive and execute Ziyou for spreading treasonous propaganda.

Nyssa finds the newly-regenerated Fifth Doctor in the Cloisters and they hurry to Tegan and Adric in the control room when the TARDIS hits an air pocket. The ship lands itself on a human colony and locks itself behind the travellers, who split into two groups, with Nyssa and Tegan booking rooms at a nearby hotel and seeing the Seventh Doctor speaking with Chris Cwej, whom Tegan quickly realises is pretending to be Australian.

The Doctor and Adric go to look for disturbances in the time field and stop at a skitrain station so that the Doctor can repair his time sensor. They meet Roz Forrester, who throws the Doctor in front of an approaching skitrain to make it automatically stop and then looks around the carriages with a time sensor of her own, followed by Adric. Two Adjudicators arrive and Roz defeats them before escaping the others by transmat. The Doctor sees ghosts and blood on his hands, leading the Adjudicators to accuse of spreading treasonous propaganda and find him guilty.

Part two
Adjudicator Provost-General Medford notices that the Doctor is taller and younger than the description he has and instead takes him and Adric to the Scientifica to make a statement. The Doctor volunteers his services as scientific advisor to Chief Scientist Juno Whitfield and learns about how terrorists led by Adam have been targeting areas frequented by the Adjudicators to protest against their presence on the planet. Later, the Doctor and Adric leave their room using the sonic screwdriver

Offended by "Bruce Jovanka", Tegan pretends to be Chris's wife to get a key to his hotel room and Nyssa goes in to investigate, but she quickly leaves upon seeing him naked. Chris later goes downstairs and asks a robot about the two of them, learning their names and identities.

The Doctor and Adric enter a room with a cryo-sleep tube inside of which a physically-degenerating person referred to only as "the Patient" is frozen. The Patient searches the Doctor's mind telepathically and learns of his birth, how he left Gallifrey with Susan and has since been travelling, but his accounts contradict one another. In a faint memory, the Doctor remembers wearing the Patient's husband's ring and promising to take her granddaughter away, but he keeps her from looking further back. The Patient regenerates.

Part three
Chris approaches Tegan at dinner and she demands an apology for his stereotyping, getting him to admit his real name and that he is undercover. He offends her again by offering his DNA to prove that he is telling the truth and then contacts Roz, who is breaking into the Scientifica, before inviting Nyssa to his room. Tegan is arrested for associating with aliens and Nyssa and Chris escape.

The Doctor and Adric are arrested as spies, but they escape Medford after five ghosts appear and kill Adjudicators with their touch. They find the Patient in the medbay and Adric surrenders to allow her and the Doctor to get away from a pursuer who turns out to be Roz, who arms him after learning that they both know the Doctor and transmats with him to the cavern. The Doctor releases Tegan from her cell and Patience, as Tegan names her, makes Contact with him, allowing him to learn about her pioneer husband and thirteen children. The trio escape the Scientifica on a skitrain, which the Doctor believes was arranged by a dangerous person whom he needs to stop.

Part four
Chris takes Nyssa to his safe house and, after telling her about the surviving Trakenites on Serenity, explains that he and Roz are fighting against corrupt Adjudicators. They travel to Skybase and sneak into a space freighter, the unloading of which is worthy of the special attention of Medford, and find boxes containing two dozen fusion devices.

The skitrain is followed by a hovercopter and the Adjudicators demand the Doctor's surrender before triggering an avalanche, burying the train. The Doctor and Patience enter a trance to save oxygen and make Contact again, sharing a memory of the House of Blyledge being raided as their son's daughter is being born. Tegan wakes the Doctor up when Quint, a Shliman, comes in search of a medallion that the Doctor took from his brother's body so that proper funeral customs could be observed and takes them aboard Adam's snowship.

Adric and Roz observe the examination of the Machine and are approached by the Seventh Doctor, who goes with Adric to a ledge and sends Roz to activate the Machine, which is one of the oldest TARDISes. She pulls a lever and the planet becomes filled with ghosts, from which the Doctor and Adric protect themselves with a chalk circle.

Part five
The Seventh Doctor tells one of the ghosts, a Ferutu, that he wishes to learn more about them and exits the circle, disappearing. Roz deactivates the Machine and intends to reactivate it in fifteen minutes, but Whitfield cuts off the air supply in her armour until she is unconscious and captures her and Adric. They examine the Machine, parts of which are familiar to Adric, and later manage to get free.

Nyssa and Chris steal the space freighter and use it to scan for a double heartbeat. Upon tracking the Doctor down, they land the freighter and Chris shoots the ice beneath a pursuing robot to stop it, but he is wounded and the box containing the fusion bombs falls down the resulting crevasse. It lands in Adam's base where the Doctor disarms them before going to the surface with Tegan and Patience and getting Chris to the Nightingale Facility. The Adjudicators attack.

Part six
The Doctor and Patience are taken to the Machine and Medford tasks them with returning it to where it came from, intending on sending the fusion bombs with it to destroy the Ferutu. They succeed, knowing that the bombs are disarmed, but Medford shoots Patience in the head to keep her from regenerating and her body disappears. The Doctor shoots him out of the room and realises, thanks to the newly-arrived Seventh Doctor, that Adam has stolen one of the fusion bombs but that both Doctors reversed the polarity of the neutron flow, meaning that Adam's bomb is defused whilst those in the Machine are not.

The Seventh Doctor explains to the Fifth that the Ferutu are all-powerful and come from a timeline in which Gallifrey was destroyed in the ancient past; to ensure this timeline, the Ferutu wipe out the Unitatus fleet above the planet. The Doctors use a shortcut through the Time Vortex to bring the Machine back along its path, attracting the attention of a Ferutu whom they trap in a circle and ask him to freeze it in the Vortex and allow both versions of history to exist. The Ferutu agrees and does so, but versions of the Machine from both timelines collide and the Ferutu's universe is destroyed, leaving only the Ferutu in the circle.

The Fifth Doctor apologises to the Ferutu for what his future self has done, although the Ferutu says that he would have done the same, and learns that he will one day meet Patience again as "love endures". When the Doctors and their companions are reunited, the Seventh Doctor says that some of the equations they have had to calculate when they save Gallifrey with their eleven other selves and the Fifth Doctor wonders if his future self has remembered everything that has happened. Roz, misunderstanding her Doctor, knocks the Fifth Doctor out with a punch.

Cast

 * The Doctor - Peter Davison
 * The Doctor - Sylvester McCoy
 * Adric - Matthew Waterhouse
 * Nyssa - Sarah Sutton
 * Tegan - Janet Fielding
 * Roz Forrester - Yasmin Bannerman
 * Chris Cwej - Travis Oliver
 * Patience - Christine Kavanagh
 * Medford / Quint / Quant / Ziyou Wanle / Ferutu - Jeremy Hitchen
 * Whitfield - Sharon Maughan
 * Adam / Falconstock / Sam / Concierge / Robot Voices - Peter Caulfield

Deviations from the original novel

 * No reference is made to the Scientifica using slave labour.
 * In the novel, the Doctor is unsure about how he was born, whereas in the audio he states that he was "unambiguously" Loom-born.
 * When the Fifth and Seventh Doctors are discussing past multi-Doctor events, they omit reference to how the Terrible Zodin used mind-rubbers, but mention the Knights of Velyshaa, (AUDIO: The Sirens of Time) which hasn't happened for the Fifth Doctor yet, and how their recent equations will be helpful when all thirteen Doctors have to work together to save Gallifrey. (TV: The Day of the Doctor) They also make reference to having "shorted out the time differential", (TV: Time Crash) the Fifth Doctor asking if he sounds older.
 * Where the novel ended with Roz knocking out the Fifth Doctor and the Seventh, Roz, and Chris departing for their TARDIS via transmat, the audio story adds an extra few moments where Adric, Nyssa and Tegan arrive while debating why the future Doctor doesn't recall these events only to find the Fifth Doctor unconscious.

Continuity

 * Nyssa mentions Trodos and the robot uprising there in 2066. (COMIC: The Trodos Tyranny)
 * The Seventh Doctor remembers joining his other twelve incarnations to save Gallifrey. (TV: The Day of the Doctor)
 * When Roz, having mistakenly believed the Fifth Doctor to the Seventh Doctor's next incarnation, questions why the Seventh hasn't regenerated, she states "You became all...." to which the Seventh Doctor replies "Frock-coat and youthful appeal?". This implies the Seventh Doctor is aware of or remembers what his next incarnation will look like. This also mirrors a similar conversation between Sarah Jane Smith and the Third Doctor in (TV: The Five Doctors)
 * The Seventh Doctor starts to mention an incident in which he and the Fifth Doctor encountered each other involving the Knights of Velyshaa, but the Fifth Doctor quickly stops him from talking, as he hasn't experienced that event yet. At that time, the Fifth Doctor was travelling with Tegan and Turlough but neither was present for that event. (AUDIO: The Sirens of Time) The Fifth and Seventh Doctors also met shortly before dropping off their respective companions Peri Brown, Erimem, Ace and Hex in Monte Carlo in 1966. (AUDIO: The Veiled Leopard) They encountered each other again in the Eighth Doctor's TARDIS (AUDIO: The Four Doctors) and in the Factory (AUDIO: The Light at the End) but neither retains any memory of those meetings.
 * When the Fifth Doctor stops the Seventh Doctor from mentioning the Knights of Velyshaa incident, he notes that they could've shorted out the time differential. He then notices that his voice sounds older than it should. (TV: Time Crash)
 * Both the Fifth and Seventh Doctors remember meeting Omega. (TV: The Three Doctors) The Seventh Doctor remembers meeting Omega on four separate occasions. (TV: Arc of Infinity; AUDIO: Omega; PROSE: The Infinity Doctors)
 * When asked about the nature of his birth by Patience, the Fifth Doctor reveals that he was "unambiguously" loom-born. (PROSE: Cat's Cradle: Time's Crucible)
 * While in telepathic contact with Patience, (TV: The Three Doctors) they reenact his memories the Fourth Doctor and Romana II about how was on around his 130th lives out of 90, (TV: The Creature from the Pit) the Third Doctor telling Kettering he has been a scientist for several "thousands y--" before being interrupted, (TV: The Mind of Evil) the Second Doctor telling Jamie and Zoe he left Gallifrey because he was bored, (TV: The War Games) the First Doctor reminiscing about Gallifrey when meeting Ian and Barabara, (TV: "An Unearthly Child") the First Doctor rescuing Patience (PROSE: Lungbarrow) and Morbius in his mindbending against the Fourth Doctor. (TV: The Brain of Morbius)
 * The Doctors both agree that if ever two of them meet again, they don't both try to reverse the polarity. Ultimately, this agreement is meaningless as the Fifth Doctor doesn't remember these events, and later the Tenth and Eleventh Doctors attempt the exact same thing on a time window. (TV: The Day of the Doctor)