As Sarah Jane and the Brigadier look on, the dying Doctor regenerates into a new, younger man with curly brown hair and beaming features. The Brigadier calls for the medical officer, Lt. Sullivan, as the Doctor starts to babble before collapsing again. Sullivan arrives to take him off while Benton brings in a routine series of reports.
At night at a secure base, a mysterious figure approaches the guard on duty, who opens fire without affect before being killed. The figure, clearly some kind of mechanical creature, breaks through the gate and confronts patrol dogs, which run off in a panic. The creature then casually rips a vault door off of its hinges and removes a single "Top Secret" red envelope.
The Brigadier discusses the disappearances of the plans, which involve a disintegrator gun, with Sarah Jane. She wonders why he's talking with her and he reveals he is missing the Doctor. Sarah Jane asks for a favor – a press pass to a think tank. An amiable Brigadier readily agrees to set her up. As they leave, the Doctor sneaks down the corridor behind them, dressed in hospital robes. He quickly finds the TARDIS and tries to get in, and searches through his clothing for the key before finding it in his right boot. Dr. Sullivan interrupts and the two verbally spar. Sullivan is baffled by the Doctor's energetic mood but determined to keep him in bed. The Doctor engages him in a round of jump-roping hopping…
The Brigadier and Sarah Jane arrive in the lab to find Sullivan tied up with a jump rope and tossed in cupboard. The TARDIS begins to dematerialize and Sarah Jane desperately pounds on the door to get him to stop. He emerges and after a few minutes manages to remember her and the Brigadier. She appeals to the Doctor to help them with the robbery of the plans.
A solitary watchman is standing guard when the mechanical figure bursts through the vault door and kills him before he can telephone for help. It then goes to a shelf and removes a specific bin full of parts.
The Brigadier bursts in to tell the Doctor about the new robbery to find Sullivan watching the Doctor go through new clothing. After several attempts the Doctor finally comes up with something resembling 20th century garb. The three of them go to the scene of the robbery and the Doctor discovers a dandelion crushed by some huge weight in excess of a quarter of a ton. Inside the Doctor immediately looks at the missing parts list and concludes they could be used to construct a disintegrator gun.
Sarah Jane arrives at the Think Tank and is introduced to the director, Hilda Winters, and her assistant Arnold Jellicoe. Meanwhile the Doctor concludes that whatever is responsible for the robberies will go for the third thing needed to build a disintegrator gun – the focusing generator. The Brigadier orders UNIT to seal the factory where the generators are made and they head for the place.
Sarah Jane is getting the standard tour when she jumps through a door marked "No admittance" and makes her way to the lab of a Professor Kettlewell. Winters and Jellicoe are dismissive of Kettlewell's work into "alternative technologies" and direct her away from a locked door. Sarah Jane slips on a patch of something wet and Winters and Jellicoe escort her out.
At the generator factory, UNIT has the place locked down tight from the ground and from above, but the Doctor points out one flaw: there is no protection for the vault from an underground approach. Sure enough, the mechanical creature burrows in from underneath and a soldier opens the vault door and opens fire. Drawn by the gunfire, the Doctor and the others investigate and find the soldier dead and a huge underground tunnel in the vault floor.
Sarah Jane talks to Professor Kettlewell and confesses how she's suspicious about Think Tank. Kettlewell denies any connection with Think Tank and she suggests they are carrying on his work in robotics. Kettlewell dismisses her concerns and gives her the brush off. Kettlewell watches as she goes back to her car and decides to go back to Think Tank before her visitor's pass expires.
Benton finds the tunnel's end and a huge pair of square footprints. He notes that it isn't a proper tunnel and that whoever dug it couldn't' have breathed. The Doctor notes this confirms his hypothesis. Meanwhile, Sarah Jane distracts the Think Tank guard with her visitor's pass then sneaks into the compound. She goes back to Kettlewell's old lab and realizes that the liquid she slipped on earlier was fresh machine oil. Then a huge robot steps into the room and demands to know why she's here. The robot advances upon her as she falls back in fear…