Is It an Emu?

Traitor

Series D, Episode 3. First broadcast on Monday, 12 October 1981.

We’re on the planet Helotrix this week, enjoying our successful subjugation of a compliant populace consisting largely of teachers and hairdressers. The occupation is ably enforced by the supremely competent Si and his régime of casually deployed neutron strikes, while Nathan reminisces pompously about past imperial glories, and Pete conducts a surprising clandestine affair with that remarkable new police commissioner Sleer. (And somewhere, in the basement, Simon is devising a galactically effective new kind of roofie.)

Until, of course, our day is ruined by a small band of barely competent outlaws, and then one of us — not sure who, to be honest — turns out to be a traitor.