The Doctor is newly regenerated and has a brief period of instability. UNIT medic Harry Sullivan tries to confine him to the sick bay, but the Doctor is adamant he's fine. When the Doctor decides to change though, his instability is reflected. First he comes out of the Tardis dressed as a Barbarian but the Brigadier reminds him UNIT is a secret organization and his clothing style might draw attention. His next few choices are also questionable, till he finally settles on the coat and scarf motif.
Meanwhile, Sarah is investigating a Think Tank organization but thinks they are hiding something. Suspicions aroused, she sneaks back in and discovers they are hiding a giant robot. The director is upset by her discovery and orders the robot to destroy her. With an established primary command of not harming humans, the robot is distressed and fails to complete his instructions. Sarah believes this causes the robot pain and she tries to comfort it.
The Doctor tries to sneak away in the Tardis but the Brigadier asks for his help and Sarah pleads he help as well. Once he hears the plans of a disintegration gun have been stolen as well as the components to build it, he snaps out of his instability and agrees to help the Brigadier. He correctly predicts the next thing needed will be a focusing generator for the weapon. Despite UNIT's involvement, the only one available is stolen.
Having befriended Professor Kettlewell, Sarah uses him to sneak into the Scientific Reform Society, the organization behind the Think Tank. They believe the world should be lead by scientists and are willing to launch a nuclear strike, planning to remain in their bunker till it's safe to re-emerge. Sarah is then shocked to learn Kettlewell is working with the SRS as well as his robot. She is saved by the timely arrival of UNIT.
Kettlewell ultimately tries to stop the robot, but is killed in the process. Having destroyed its creator, the Robot goes mad. The Brigadier learns it's not as hard to destroy as he thought and shooting it with the disintegrating gun only makes it grow in size. Enraged, the giant robot grabs Sarah to 'protect' her as she was the only person who tried to befriend her. It storms off to attack UNIT.
The Doctor saves the day when he uses another of Professor Kettlewells inventions to destroy the robot. With Harry Sullivan's help, he recreates a virus that eats away at metal and breaks it down. The virus is thrown onto the robot and quickly spreads till the robot is destroyed, much to the dismay of Sarah. The Doctor offers to take Sarah on a trip in the Tardis to cheer her up, but when Harry catches them sneaking off he is invited to go along as well.
WHO: Harry Sullivan
PLANET OF ORIGIN: Earth, 1974
REASON FOR JOINING: Invited into the Tardis to prove it can't travel in time and space.
NEXT: Enter the Kaleds.