Parts: 4
Writer: Johnny Byrne
Airdates: 3 January 1983 - 12 January 1983
Production Code: 6E

Doctor: fifth
Companions: Nyssa, Tegan

Megalomaniac of the Week: Omega
Special Guests: the Time Lords

Story: There is a traitor among the Time Lords. The bitter and vengeful Omega, trapped in the antimatter dimension, desires to return to the normal universe - and to do so, he needs the biodata of a Time Lord. The Doctor turns out to be the perfect match, so Omega's servant delivers it.

The Doctor and Nyssa have just finished repairing the scanner's audio function when something from another dimension approaches the TARDIS. As the Doctor and Nyssa flee, it breaches the TARDIS' defenses and attempts a temporal bonding with the Doctor. It fails, but now has a foothold in the normal universe. Nyssa learns that they are in a region of space known as the Arc of Infinity - a gateway between the normal and antimatter universes. Meanwhile, Robin Stuart and his friend Colin Frazer are on holiday in Amsterdam. Despite Colin's uneasiness, they spend the night in a crypt - but during the night, Colin hears the sound of a TARDIS materializing. When he investigates, he finds a tall, vaguely avian creature - the Ergon - which fires a weapon at him that causes him to disappear. Robin narrowly escapes the same fate, and when he later returns to the crypt, he finds that Colin has become a zombie.

Having just lost her job, Tegan Jovanka arrives in Amsterdam to see her cousin Colin. When Robin tells her what has happened to him, Tegan decides to get to the bottom of the matter. Meanwhile, the Doctor and Nyssa realize that they must find and destroy the antimatter creature in order to prevent it from bonding with the Doctor. But the Time Lords have already analyzed the situation and found that the creature is shielded and they cannot locate it. So they come up with a much simpler solution - kill the Doctor...