Again, while this is by no means a classic episode it moves faster than any of the individual episodes of the previous serial, while finding time for glorious character moments between Troughton and Padbury and revealing the monsters ready for the second half of the story.

This episode focuses almost entirely on the regulars: while Holmes will go on to develop a talent for vivid supporting characters, this feels like the right choice. Troughton and Padbury get the best scenes: as the Doctor and Selris go to investigate the roots of the Krotons’ organic spacecraft, Zoe decides to try out the learning machines rather than waiting about obediently. Obviously, she aces the tests: her genius is ‘very irritating at times’, especially when it means the Doctor has to take a more direct approach to facing the enemy than it seems he was planning. The subsequent sequence of the Doctor taking the Krotons’ tests is a comic and character highlight – him: flustered, absent-minded brilliance; Zoe: gentle chiding as he mucks up the first few questions. It’s utterly charming and prefigures the fourth Doctor and Romana relationship.

The following trip into the Kroton spaceship is an opportunity for director David Maloney to show off a bit: disorienting closeups with fish-eye lenses conveys the sense of the Krotons mentally attacking the Doctor and Zoe. The resultant brain drain feeds into a bubbling vat of ‘primordial soup’ (which means, knowing Holmes, that Kroton is a play on crouton, given they’re floating in the soup). The monsters look a bit disappointing: bulky, clumsy and a weird mish-mash of rubber skirt, crystalline body and robot arms. However, their dialogue suggests they are frighteningly smart: they easily deduce that the Doctor and Zoe aren’t Gonds but ‘a similar biped animal but not from this planet’: human beings described like livestock is another Holmes hallmark.

Next episode: “The Krotons” – Episode Three