In the serial, the alien time traveller the Sixth Doctor (Colin Baker) is put on trial by his people, the Time Lords, and is accused of meddling in the affairs of the planet Thoros Beta in the 24th century. Much of the story consists of a video testimony presented by the prosecutor - the Valeyard (Michael Jayston) - of the Doctor discovering the alien arms dealer Sil (Nabil Shaban) is looking for a way to prolong the life of Lord Kiv (Christopher Ryan) by transplanting Kiv's mind into another.