The giant robot bears down on the Doctor but Sarah Jane arrives and intervenes, pleading for his life. Sarah tries to convince the robot it's been reprogrammed and it begins to wander in confusion, saying it's in pain. Benton and the other UNIT soldiers barge in and open fire and the robot barges out, unaffected by the bullets. Benton checks in on Sarah Jane who is trying to revive an unconscious Doctor, and finds Kettlewell bound and shoved in a closet.
Back at UNIT, Kettlewell admits Winters and Jellicoe have reprogrammed the robot and talks about the living metal it's made out of, that can grow like a living organism. He also talks about a virus he created that can break down all types of metal. Kettlewell notices the Reform Society brochure and Sarah Jane seizes on that to help her get in with tonight's meeting. When Benton tries to stop them, Sarah Jane gives him a hard time and insists since they're not UNIT members he can't do anything.
At the Society meeting Kettlewell shows his invitation and gets in. The Brigadier finds out and isn't happy, but hasn't got permission to raid the Think Tank. The Doctor concludes that the murdered man, Chambers was guarding some major secret and the Brigadier admits that Chambers was holding the key codes for the world powers' nuclear missiles. When the Doctor hears that Kettlewell went with Sarah, he points out that Kettlewell is on the whole thing.
Kettlewell lets Sarah Jane in through a back door and she sneaks into the meting where Winters is giving a speech to her followers. She brings in Kettlewell and starts praising him while the robot comes out.
The Doctor tries to bluff his way past the guard and trips him with his scarf when it doesn't work. Winters turns the robot loose on Sarah Jane but the Doctor barges in and distracts the audience with card tricks and a dance routine. The guards manage to get their hands on the Doctor, who confronts Kettlewell. The professor admits he wants the power to get the world to listen to him, and Kettlewell blanches when he realises Winters plans to have the Doctor. The Brigadier barges in and Winters escapes with Sarah Jane, Jellicoe, and Kettlewell, using the robot for cover.
While the Doctor wonders where Winters might go to survive a nuclear holocaust, Sullivan contacts the Brigadier and mentions something about a bunker before Jellicoe knocks him unconscious. The Doctor heads out with UNIT to the Think Tank's Cold War atomic shelter. Winters activates the automated defense system but the Doctor spots an automated machinegun and everyone gets to cover just in time.
Winters calls to inform the Brigadier they have 30 minutes to surrender or they'll unleash the missiles. The robot is guarding Harry and Sarah Jane is brought in. The UNIT soldiers destroy the automated machineguns with grenades but the Doctor realises something is amiss and uses his sonic screwdriver to detonate a series of hidden mines. They get to the bunker door and the Doctor converts his sonic screwdriver to a miniature sonic lance to burn through the lock.
Jellicoe and Kettlewell start to panic but Winters orders Kettlewell to prepare the launch codes. Jellicoe gives the robot the disintegrator gun and sends it to engage the UNIT troops. They fall back as the robot emerges and disintegrates a soldier. The Brigadier calls up a tank which the robot disintegrates with equal ease. It warns the troops to withdraw or it will destroy them all!