Snatched out of time and space and brought before the Time Lords on Gallifrey, the Sixth Doctor is on trial for his life. While the Doctor asserts that the evidence of the Matrix, the repository of all Time Lord knowledge, has been tampered with, the mysterious and vengeful prosecuting council, the Valeyard, is confident that the Doctor will be sentenced to death. In a dramatic intervention the Valeyard's true identity is revealed but he escapes from the courtroom into the Matrix, and it is into this nightmare world that the Doctor must follow – to face his ultimate foe …