On top of the killer rehabilitation machine, the World Peace Conference, the missile transport we now have a prison break and the Master thrown into the mix on the basis that something has to stick. The risk is that there are a lot of stories rather than one good one, but Houghton has more or less proved he can keep the plates spinning, and at least this is giving the sense that everything – somehow – is connected, rather than being introduced, Terry Nation style, to extend the script for another week.

The reintroduction of the Master in particular is a neat surprise – dropped in the middle of the episode rather than at the soon-to-be tediously familiar Part One cliffhanger. He’s still wearing his telephone engineer disguise from Terror of the Autons, but, brilliantly, he removes the mask and coveralls to reveal an immaculate business suit. And rather than having a secret base he issues his order from the back of a chauffeur-driven limo. For a certain type of fan, this is the ultimate version of the Master: a suave and sophisticated villain rather than an insanely desperate survivor. His presence starts to join the dots between the other plots – he’s controlling Chin Lee, who is also connected to the Keller Machine and who, at the cliffhanger, invokes its power in an assassination attempt on the American delegate.

The Master isn’t the only call-back to previous episodes. Benton gets to dress in civvies and trail a suspect, just like he did in The Invasion. And Houghton inserts a reference to Inferno, with the Doctor’s greatest fear seeing the world consumed by fire. Pertwee’s performance of genuine terror is surprising – we’ve never seen the Doctor quite so emotionally vulnerable before (Jo has to calm him down, although she saves her Dairy Box, the go-to sweets for the sick and dying, for Barnham). My only regret is, although William Marlowe is very good as the hardened criminal Mailer, that they didn’t get William Dysart back to play Reegan from The Ambassadors of Death, sentenced to serve his time for his role in the Carrington Plot.

The Doctor again gets to be the errant schoolboy in the Brigadier’s class, refusing Mike Yates’ order for him to return to London (in a rather contrived and unlikely moment he uses Venusian karate against Mike, as if the show wants to set up some conflict between the two), and enjoys winding the Brigadier up. In general, he rails against ‘idiots in authority’ and reveals he knows Mao Zedong well. And to think back in the 1990s when it was trendy to bash Pertwee someone claimed “the third Doctor is a Tory!”. What rot. As if his Malcolm Hulke ideals and the Red Flag lining of his cape weren’t clue enough, it’s made plain here: the third Doctor is a Marxist.

Next episode: The Mind of Evil – Episode Three