The TARDIS is drawn to a space station where the Doctor is subjected to a Time Lord inquiry into his behaviour, presided over by an Inquisitor. The prosecuting counsel, the Valeyard, presents the first piece of his evidence, which consists of a recording played back on a screen linked to the Matrix. It concerns a visit by the Doctor and Peri to the desolate planet Ravolox, which turns out to be a future Earth, shifted light-years through space.
The Valeyard’s second segment of evidence relates to the planet Thoros-Beta. Here the Doctor and Peri meet their old adversary Sil and others of his Mentor race, whose leader Kiv is awaiting an operation from a scientist named Crozier to transplant his brain into another body. They also form an uneasy alliance with a kidnapped Krontep warrior, King Yrcanos, and encounter a group of resistance fighters.
The distraught Doctor gives the court his evidence for the defence. He chooses an incident from his own future, in which he and his companion Mel arrive on the space liner Hyperion III in response to a distress call.
With the evidence complete, the Doctor learns that the Master has gained illicit access to the Matrix in his TARDIS.