Parts: 4
Writers: Pip and Jane Baker
Airdates: 1 November 1986 - 22 November 1986
Production Code: 7C
Alternate Titles: The Trial of a Time Lord parts Nine through Twelve; The Ultimate Foe
Megalomaniac of the Week: the Valeyard
Creature Feature: Vervoids
Special Guests: the Time Lords
Story: The Doctor begins his defense with an adventure set in his future, the year 2986, on board the Hyperion III - a space liner carrying a cargo of precious metals from the planet Mogar to Earth. One of the passengers is hiding a dangerous secret, and will kill to protect it.
The TARDIS receives a mayday call, and as Mel points out, they are bound to answer it. But the captain of the ship is wary of having the Doctor on board, as he usually brings trouble with him, so the Doctor and Mel are confined to passenger quarters. But, as Mel points out, this still affords them a great deal of freedom, so while the Doctor relaxes in the lounge, she attempts to make contact with whomever summoned them. While her attempt proves successful, and she and the Doctor are directed to a meeting in cabin six, by the time they arrive, the cabin has been ransacked. And soon the stakes are raised - one of the passengers is murdered, leaving a variety of suspects.
But murder is not the only dark thing afoot on the liner. Secrets seem to lurk behind every corner: what is being hidden in the isolation lab? Are all of the passengers quite what they seem? And most importantly, what dangers lie dormant in the hydroponics bay? Soon the presence of a single murderer is only one of the passengers' worries, for all of them are threatened by an even greater menace. It's up to the Doctor and Mel to poke their noses into every nook and cranny to solve the mystery...before the entire space liner becomes nothing but compost, and all of animalkind is annihilated by the deadly thorns of the Vervoids.