The Secret History (audio story)

 celebrated the 200th release in Big Finish Productions' monthly range. It partnered the Fifth Doctor with First Doctor companions Vicki Pallister and Steven Taylor, and concluded the "Locum Doctors" trilogy.

Publisher's summary
The TARDIS brings the Doctor, Steven and Vicki to the Italian city of Ravenna in the year 540 – besieged by the army of the celebrated Byzantine general Belisarius. Caught up in the fighting, Steven ends up on a boat bound for Constantinople, the heart of the Roman Empire.

Rescuing Steven, however, is the least of the Doctor's problems – because he shouldn't be mixed up in this particular adventure at all. Someone has sabotaged his own personal timeline, putting him in the place of his First incarnation... but who, and why? The truth is about to be revealed – but at what cost to all of the Doctors, and to the whole future history of the planet Earth?

The Gothic War (1)
Vicki Pallister and Steven Taylor are running from some aliens, who are shooting at them. They reach the TARDIS, where the Doctor is waiting for them. Inside, however, they find a stranger instead of the Doctor. The stranger greets them by name, expressing how good it is to see them again. To their shock, he reveals himself to be the Doctor.

Steven and Vicki don't believe him, as he looks nothing like the First Doctor. The aliens start banging on the doors. The Fifth Doctor asks what's banging on the doors. Steven and Vicki tell him, prompting him to move to the control panel and dematerialise the TARDIS, to the surprise of Steven, as only the Doctor knows how to use the TARDIS. The two are now considering that the Fifth Doctor might be telling the truth. He explains that he has somehow switched places with the First Doctor. The duo mistakenly assume the Fifth Doctor is an earlier version of the First Doctor, but quickly realise that's incorrect, as he knows who they are. The Doctor then references past adventures, fully convincing the two that he is the Doctor. He also realises where in his past he is, based on what has just happened. Vicki asks him why he is here, instead of the First Doctor, but the Fifth Doctor has no idea. He explains he was just in his own TARDIS, then appeared in this one.

The TARDIS lands, and the Doctor already knows where they are: Ravenna, Italy, 540. The Doctor says they should go outside, but is hesitant. He explains that he remembers how this adventure turns out, but as soon as he leaves the TARDIS, the timeline will change to one in which he didn't experience these events in his first incarnation, and thus he'll forget everything that's about to happen. It's crucial that he does things as the First Doctor would do them, otherwise he'll have interfered with his own timeline, which could have severe consequences. As they go through this adventure, the Fifth Doctor will do everything he can to figure out what caused him to switch places with the First Doctor, and how to reverse the switch.

They step outside and the Doctor gives some historical background about Ravenna and the split of the Roman Empire. The city is about to be sieged by the Romans, as the trio have arrived during the Gothic War.

Procopius, advisor of Flavius Belisarius is against the idea of Belisarius going into the city. Belisarius reminds him that he needs to go in to fool the people into thinking that the Romans are willing to end things peacefully. He tells Procopius that he will be going into the city with him. Procopius convinces Belisarius to bring Quintis with them.

The Doctor watches Belisarius enter the city and head to the palace. He and his companions follow the crowds there. While this is happening, Procopius notices that Quintis has gone missing.

Quintis joins Sophia in her room and they go onto the balcony. Quintis gives her a crossbow, and tells her to protect the Doctor, Vicki and Steven.

Belisarius enters the palace and claims the city. Procopius goes outside and informs the crowd about what has happened, and that if they cooperate, they will be left alone. The people take up a cry of "No surrender!" and the Doctor and his companions move back towards the TARDIS.

Quintis and Sophia watch this from the balcony. Quintis leaves, after telling Sophia that it's vital that the Doctor isn't killed, as he's not supposed to die here.

Steven gets separated from the Doctor and Vicki, as they try to get through the crowd. The Doctor is confused, as the people are supposed to be surrendering. A man confronts the Doctor, and is about to attack him as a traitor to Ravenna when he is shot by Sophia's crossbow.

Meanwhile, Steven accidentally offends another man, who knocks him out.

Vicki and the Doctor get to the TARDIS, and find Procopius hiding behind it. The Doctor goes back into the crowd to look for Steven, and Procopius recounts the story of the capture of Carthage to Vicki.

Steven is woken up by a physician, who is confused by Steven's requests for the Doctor.

The Doctor returns to Vicki and Procopius, unsuccessful. Procopius suggests that Steven might have been mistaken for a Goth, prompting the Doctor to ask for an audience with Belisarius. Procopius takes him for said audience, while Vicki continues the search in the crowd.

Steven asks where he's being taken, but the physician assures him that he'll take care of him.

The Doctor is granted an audience with Belisarius, who tells the Doctor that all Goths were put on a boat to Constantinople. The boat can't be brought back without a messenger being sent out, which will take time, and no one is allowed to enter or leave the city until Belisarius says otherwise.

Vicki finds a woman who has seen Steven been put on a boat for Constantinople with the other injured. As a cover, Vicki says she and Steven have eloped, and the woman, Sophia, is going to Constantinople soon. She's from Greece, and she left there with a stranger who understood her.

Steven wakes up again, and the physician who rescued him identifies himself as Quintis. They're on a ship to Constantinople, and when they get there Quintis will explain to the captain that Steven isn't a Goth, so he will be free, but still stranded.

A message arrives for Belisarius from Emperor Justinian. The emperor wants him to end the siege and return because Constantinople is under threat from the Persians. Belisarius refuses, but the Doctor convinces him otherwise. Procopius agrees to let the Doctor and Vicki travel with them.

At the port, Belisarius refuses to bring the TARDIS on board. The Doctor explains to Vicki he only changed Belisarius's mind about staying in Ravenna, because he knew it was supposed to happen that way. Vicki spots Sophia in the port.

Steven is amazed by Constantinople. Quintis asks him what he plans to do. Steven intends to secure passage back to Ravenna, but he has no money. Quintis tells him he can find employment at the chariot races, with his friend Yazid.

The Doctor and Vicki arrive in the city. As they leave the ship, Imperial guards identify the Doctor by name and arrest him.

Quintis meets with Justinian. He has told the emperor that the Doctor is a Persian spy, and Justinian will have him put to death, to the delight of Quintis.

The Last Roman (2)
to be added

The Catastrophe Cure (3)
to be added

The Twist of Fate (4)
to be added

Cast

 * The Doctor - Peter Davison
 * Vicki - Maureen O'Brien
 * Steven - Peter Purves
 * Sophia - Lysette Anthony
 * Theodora - Sarah Woodward
 * Procopius / Yazid - Tony Millan
 * Belisarius - Giles Watling
 * Justinian - Tim Wallers
 * The Monk - Graeme Garden (uncredited)

Continuity

 * The Doctor refers to when he told Vicki to give Barbara Wright another chance after Barbara shot Sandy on Dido (TV: The Chase) and their trip to Rome in 64. (TV: The Romans)
 * The Doctor tells Vicki that he cannot change history, because if he did the Roman Empire may survive to her day. His fourth incarnation encountered an alternative Roman Empire in the 1970s. (COMIC: The Iron Legion)
 * The Doctor is surprised he is the only one with a key to the TARDIS, and says that Vicki and Steven should have keys of their own. He would later grant keys to companions such as Liv Chenka and Rose Tyler, and had previously given one to Susan Foreman. (AUDIO: A Life in the Day; TV: Aliens of London, The Daleks)
 * Steven and Vicki do not initially recognise the Monk as he has changed his appearance from the last time they encountered him. (TV: The Time Meddler)
 * The Monk has decided to intervene in the Doctor's past to remove him from existence, because through the Doctor he lost someone very close to him (AUDIO: To the Death). The Monk decided to intervene in certain points in the Doctor's life in the process of returning to his first incarnation (AUDIO: The Defectors, Last of the Cybermen) and take control of his timestream, removing the Doctor from existence. The Great Intelligence would later attempt a similar aim. (TV: The Name of the Doctor)
 * The Monk believes the First Doctor would have done nothing to alter history. (TV: The Aztecs)
 * Steven would meet the Fifth Doctor again in an alternative Death Zone. (AUDIO: The Five Companions)