Parts: 4
Writers: Bob Baker and Dave Martin
Airdates: 30 December 1972 - 20 January 1973
Production Code: RRR

Doctors: first, second, third
Companion: Jo

Megalomaniac of the Week: Omega
Special Guests: UNIT (the Brigadier, Benton), the Time Lords

Story: When a weather-balloon-type object for studying cosmic rays comes down to Earth in a bird sanctuary, the scientist in charge, Dr Tyler, is called to collect it. But just as he arrives, the game warden who had been keeping watch over the device disappears in a flash of light. Concerned, Tyler takes the device to UNIT and describes to them what happened. While the Doctor and Jo go off to investigate, he begins working with the device...but soon he, too, has disappeared.

A multicolored blob of energy emerges from inside the device and goes down the sink drain. It reappears out of a drain on the street near Bessie, and as the Doctor and Jo watch, it grows in size, touches Bessie - which vanishes - and then retreats back down the drain. The Doctor deduces that the organism is hunting him, and soon reinforcements - tall, blobby creatures that cannot be destroyed - appear and begin mowing down UNIT soldiers.

Confronted by an enemy he does not understand and cannot combat, the Doctor calls on the Time Lords for help. They discover that the creature has been sent to Earth from an anti-matter universe by a being with powers equal and opposite to their own. As all their resources are being diverted to battling the anti-matter being, the only help they can send the Doctor is...himself. With their combined intellects, the three Doctors are all that stand between one of the greatest figures in Time Lord history and the annihilation of the entire universe.