Robot

Robot was the first serial of season 12 of Doctor Who. This story had numerous notable firsts and lasts.

In terms of firsts, Robot was Baker's debut episode as the Fourth Doctor, which followed on directly from the end of Planet of the Spiders. It was also the first story to feature new companion Harry Sullivan, played by Ian Marter. It was the first story for which Robert Holmes was the script editor.

In terms of lasts, this was the final story produced by Barry Letts. It was the last story in which Nicholas Courtney had a regular role as Brigadier Lethbridge-Stewart, as the character did not appear again until Terror of the Zygons the following season. After that, he was entirely absent from the series until Mawdryn Undead in 1983. Likewise, John Levene's newly promoted Warrant Officer John Benton made his final appearance as a regular character. His only subsequent appearances were in the aforementioned Terror of the Zygons and The Android Invasion.

Synopsis
Trouble is brewing once again; top secret plans are being stolen with their guards killed. All evidence seems to point to the culprit being a sentient robot created by a think tank; however, his basic programming prevents him from killing, providing a contradiction to the clues.

At the same time, the Doctor is recovering from his latest regeneration; can he regain his senses and help UNIT solve the case before time runs out?