Robot is the first serial of the 12th season in the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 28 December 1974 to 18 January 1975. It was the first full serial to feature Tom Baker as the Fourth Doctor, as well as Ian Marter as new companion Harry Sullivan. In the serial, the director of an English research institute plots to use an experimental robot to steal nuclear launch codes and blackmail the world's governments with them.

The serial brought a full end to the Pertwee era, as it was the final story with the production team of Barry Letts and script editor Terrance Dicks. It was also the final regular appearance of UNIT, who had become regulars starting with the first Jon Pertwee serial Spearhead from Space.

The episode received mixed reviews from critics, seeing praise for the acting, especially that of Baker and Marter, but criticisms due to stereotypical villains and a weak plot.

Following his regeneration, the Fourth Doctor becomes delirious and erratic. The Doctor tries to sneak off in his TARDIS, but the Brigadier and Sarah Jane stop him, convincing him to help in finding the culprit in the theft of top-secret plans for a disintegrator gun.

Sarah investigates the National Institute for Advanced Scientific Research, colloquially known as the "Think Tank". She finds that director Hilda Winters, her assistant Arnold Jellicoe, and Professor J.P. Kettlewell are developing a robot, K1, to be used to perform tasks in hazardous locations. Winters and Jellicoe have secretly instructed K1 to kill Cabinet Minister Joseph Chambers, and use a completed disintegrator gun to steal international nuclear launch codes from Chambers' safe. K1 discovers Sarah's presence, and Winters orders K1 to kill her. When UNIT arrives, the three conspirators and K1 escape with Sarah as their hostage.

Winters sends a list of demands to the world governments and gives them thirty minutes to comply, then orders Kettlewell to connect to the launch computers. Kettlewell, who never expected their plan to get to this stage, hesitates, and in the ensuing discussion, Sarah and Harry attempt to escape with Kettlewell's assistance. Winters orders K1 to stop them, but the robot inadvertently fires the disintegrator gun at Kettlewell, killing him. In a confused state due to the death of its creator, K1 falls to the ground and apparently shuts down.

As UNIT forces take Winters and Jellicoe away, K1 reactivates and attacks UNIT. K1 seeks out Sarah to protect her, a result of an Oedipus complex it developed from Sarah's previous compassion, according to the Doctor. The Doctor finds out about the living metal that Kettlewell used in constructing K1 and the metal virus he designed to reduce the world's metallic waste. He races back to Kettlewell's lab to synthesise a batch of the virus. The Brigadier fires the disintegrator gun at the robot, but the blast is absorbed by the living metal- and K1 grows to an enormous size. The Doctor returns, throwing a bucket of virus solution onto K1; the robot slowly shrinks before the virus consumes it. As they regroup back at UNIT headquarters, Sarah is saddened by the loss of K1. The Doctor offers to cheer her up with a trip in the TARDIS, extending the invitation to Harry as well.

As the Doctor was transitioning from the third to the fourth incarnations, changes were also occurring in the production department of Doctor Who. Barry Letts, who had been the producer since the second Jon Pertwee serial in 1970, was leaving the series, but would stay on to cast the part of the new Doctor as well as produce this debut serial. Letts would be succeeded for the next story by Philip Hinchcliffe, who trailed him on this story.