The focus of the episode is on the unravelling conspiracy’s increasingly murderous attempts to maintain its cover up, facilitated by Reegan’s cruel self-interest. It’s best illustrated in the exchange between Liz and Lennox (Cyril Shaps), when Liz essentially accuses him of being an accessory to murder, prompting the fussy little worm to turn and sell out to the Brigadier and UNIT. In response, the conspiracy arranges for a mystery UNIT member to lock Lennox in a cell with a radioactive isotope, ironically turning his own work against him in a way that has all the hallmarks of Reegan’s sadism.

Reegan is a great character. He’s a petty villain, by Doctor Who standards, but he has as sarcastic a sense of humour and chip-on-his-shoulder as a Robert Holmes baddie: when Liz worries that he’s arranged for the Brigadier to be killed Reegan replies, ‘No, just some of the other ranks.’ He’s also clearly making his own plans for the alien ambassadors, above and beyond the conspiracy. And he’s not afraid to get his hands dirty, going undercover as a mechanic to infiltrate the space centre and sabotage the Doctor’s rocket (which results in some classic Pertwee gurning).

Fortunately, the regulars are written equally strongly. Caroline John gets some great material as Liz, as she’s defiant towards Regan’s heavies (‘It’s all right I won’t hurt you’) and persuades Lennox to do the right thing. Having learned nothing from the Silurians, the Doctor is adamant that the aliens are being used by someone – still giving extra-terrestrial life forms the benefit of the doubt. The Brigadier shows a sweeter side as he makes the effort to see the Doctor off before his trip into orbit: a lovely, understated moment of intimacy between the two of them. And Sergeant Benton is back!

Next episode: The Ambassadors of Death – Episode 6