Following on from The Mysterious Planet, the Valeyard and the Doctor argue about the Doctor’s involvement in those events. The Inquisitor warns them both to pay due respect to the judicial process. The Valeyard proceeds to present his second block of evidence - the Doctor's arrival on the planet Thoros Beta.
The TARDIS arrives on the planet, where the Doctor shows Peri a weapon given to him by the "Warlord of Thordon", made on Thoros Beta. He states that has come to find out how the warlords obtained such technology. They enter a cave, where Peri is grabbed by a large monstrous creature, which during a struggle the Doctor shoots with the gun.
The Valeyard accuses the Doctor of deliberately shooting the monster, but he replies that the weapon went off accidentally.
A figure arrives and accuses the Doctor and Peri of murdering the Raak, despite their protestations that it attacked them first. The figure asks if they are part of Crozier's group, and the Doctor says that he is. They flee before they can be identified as imposters, but are quickly faced by another monster, but it reacts kindly when the Doctor is nice to it. They are forced to flee further, and as they hide they see three reptilian figures being carried along by guards, the third of the figures is shown to be their old enemy Sil. The Doctor realises that Sil is probably behind the arms sales, and informs Peri that Thoros Beta is the home world of Sil's race, the Mentors.
In Crozier's laboratory, King Yrcanos is being experimented on, and the Doctor and Peri sneak inside. As the Doctor sabotages some of Kiv's equipment, Sil arrives in the laboratory. The Doctor is strapped to a table, and Crozier applies a metal helmet to his head. Crozier states that the equipment to extract the truth from a suspect, and that technique could prove fatal. He starts to probe the Doctor's mind, but Yrcanos awakes and destroys the equipment. Overpowering the guards he departs the laboratory, followed by a stunned Doctor and Peri. Yrcanos outlines his plans to attack the Mentors. The Doctor says he would enjoy that, and then collapses.
The Doctor tells the Inquisitor that he cannot remember these events. The Valeyard tells him he is in for a surprise if this is true.
Yrcanos, the Doctor and Peri go to where new slaves are brought into the base. Yrcanos plans to attack the guards and steal their weapons, but as he sneaks into the room, the Doctor calls out to the guards, giving him away. Yrcanos, unable to fight the guards, flees. Peri points a weapon at Sil, and asks the Doctor for help, but he ignores her. Peri drops the weapon and flees after Yrcanos. Sil asks the Doctor why he helped the Mentors, and he replies that the odds were on their side.
The Doctor insists that the footage is not of him, but the Valeyard tells him that the Matrix cannot lie.
Peri comes across Matrona, who allows her to join the Mentors' servants rather than turn her over to the guards. Covered with a veil, she enters the Commerce Room with Kiv's medication. The Doctor calls to her to get him a drink, so she disguises her voice to avoid being recognised. When she brings him a new drink, the Doctor uncovers her and denounces her as an enemy to the Mentors.
The Doctor tells the Courtroom that what they are seeing is all part of his ploy. He says he planned to gain the Mentors' trust so that he would be allowed to interrogate her alone, giving them a chance to escape.
Peri is lashed to rocks on the shoreline and the Doctor stands over her, accusing of being a spy. She asks why he is behaving the way he is, and the Doctor tells her that Crozier is planning to put Kiv's brain into his body unless he can help them. Crozier stops the interegation, saying that they have more effective methods of extracting the truth from Peri. As they re-enter the complex, Yrcanos attacks the guard, and threatens to kill the Doctor. However, Peri smashes the gun from Yrcanos's hands allowing the Doctor to flee. In Crozier's laboratory, the scientist prepares to transplant Kiv's brain into a recently deceased Mentor corpse with the help of The Doctor. The operation proves successful.
Meanwhile, Yrcanos, Peri and Dorf team up with members of the Alphan resistance. Agreeing to allow Yrcanos to lead them in an attack on the Mentors, they go to the resistance arms dump, but they are ambushed by Mentor guards and shot down. However, it is revealed they have merely been stunned, and they are taken to cells.
In Crozier's laboratory, Lord Kiv is rambling due to the body of the fisherman influencing his brain. Crozier makes plans to transfer the brain into another more suitable body, and suggests using Peri. The Doctor says he would prefer that she is not experimented on, but while he is trying to find another candidate, Peri is brought to the laboratory, and strapped to the operating table. Crozier begins to prepare her for the surgery.
The Doctor goes to Yrcanos's cell and tricks the guard allowing Yrcanos and Dorf to escape. Together they free the remaining resistance members. They head towards the control room from where all the slaves are mentally controlled and succeed in freeing the slaves from mental control, but Dorf is killed by a passing guard. Lord Kiv is taken to the laboratory to prepare for the operation. As the Doctor heads towards the lab, he is summoned by the Time Lords and promptly vanishes.
The Inquisitor tells the Doctor this was the result of an order from the High Council, because the result of Crozier's experiment would affect all life in the Universe.
As Yrcanos prepares his attack on the laboratory, the Time Lords capture him in a time bubble so that his attack is perfectly timed. When Kiv awakes in Peri's bald body, the time bubble dissipates and Yrcanos bursts into the laboratory. He is consumed with fury and begins firing his gun wildly.
The Doctor is shocked by what he has seen. The Inquisitor and the Valeyard tell him that it was necessary to end Peri's life to prevent the disastrous consequences of Crozier's experiment. The Doctor insists that he was fetched out of time for some other reason, and he is going to find out what.
Doctor Colin Baker (Sixth Doctor)
Companion Nicola Bryant (Peri Brown)
Lynda Bellingham — The Inquisitor
Michael Jayston — The Valeyard
Brian Blessed — King Yrcanos
Nabil Shaban — Sil
Christopher Ryan — Kiv
Patrick Ryecart — Crozier
Thomas Branch — The Lukoser/Dorff
Gordon Warnecke — Tuza
Alibe Parsons — Matrona Kani
Trevor Laird — Frax
Richard Henry — Mentor
Writer Philip Martin
Director Ron Jones
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 7B
Series Season 23
Length 4 episodes, 25 minutes each
Originally broadcast October 4–October 25, 1986