Parts: 4
Writer: Terrance Dicks
Airdates: 28 December 1974 - 18 January 1975
Production Code: 4A

Doctor: fourth
Companions: Sarah, Harry

Creature Feature: K-1 Robot
Special Guests: UNIT (the Brigadier, Benton)

Story: The Brigadier and Sarah watch in awe as the Doctor regenerates into a completely new body. When the Doctor awakes, he seems quite disoriented, so the Brigadier puts him in the capable hands of Doctor Harry Sullivan. The Doctor, however, has no intention of staying under medical care, and sneaks back to the TARDIS as quickly as he can. Only Sarah's pleas for him to stay and help UNIT prevent him from immediately taking off - plus, he is intrigued by the Brigadier's tale of top-secret weapons theft. First the plans for a disintegrator weapon were stolen, then the components, and UNIT must find the thief before they obtain the third and final part.

Meanwhile, Sarah pulls UNIT's strings to obtain a journalistic pass into the prestigious Think Tank, where high-level scientific research is conducted. Despite the disapproving presence of director Miss Winters, Sarah slips into as many nooks and crannies as she can - and one of these is the cleared-out laboratory of a Professor Kettlewell, robotics specialist. Suspecting that Miss Winters has concealed something, Sarah returns later in the day and sneaks back in. On the floor of the laboratory is a puddle of oil, and while Sarah ponders the implications, a robot lurches out of the shadows toward her...

Kettlewell's robot has fallen under the control of an organization that is determined to bring the world under their influence...even if they have to start a nuclear armageddon to do it. Without the new Doctor's quirky brilliance and Sarah's compassion, even UNIT doesn't stand a chance...and that's before the robot literally becomes an even bigger problem.