The long recap of the fight sequence from the end of Episode Two indicates that this is going to be a bit thin, and the script is mostly just vamping, spending loads of time getting the Doctor out of handcuffs; Trenchard delaying Captain Hart’s investigations, and a submarine in danger from the monsters we already know are behind the missing boats. In theory, this is infamous middle-episode padding.

In practice, it’s a lot more fun than a synopsis sounds, largely because the regulars are all brilliant and Malcolm Hulke seems to be amusing himself with a parody of the “trapped on Earth” format he so disliked. Instead of the trapped Doctor reluctantly working with the Brigadier to defend Britain against enemy aliens we have the trapped Master reluctantly working with Trenchard to (so he says) defend Britain against enemy aliens.

Consider the scene where the Master is tinkering with a gadget in his makeshift lab, grouching at a subordinate sent to summon him to Trenchard’s office. It’s exactly the kind of scene we’ve seen played out before between the Doctor, Benton and the Brigadier. The Master’s high-handed attitude towards Trenchard mirror’s the Doctor’s towards the Brigadier, and Trenchard’s attempts to buy time for his eccentric Time Lord associate exactly match all those times the Brigadier has provided cover for the Doctor against meddling civil servants or UNIT high command.

With Roger Delgado playing the Doctor’s traditional role, Jon Pertwee and Katy Manning get to have fun for a week. Pertwee delivers some great one-liners (‘How very kind of you… How very unkind of you’ when a guard uncuffs him only to secure him to a chair; ‘I’m very relieved to hear it’ when he’s told the guard will be posted outside the door to his cell). His sheer joy when Jo appears at the window is wonderful, and their business with the escape is perfectly done, showing again how well rehearsed and sensitively directed this is. Plus, the cliffhanger of the Sea Devils rising from the water is so iconic the show repeated it at least twice with the Marshmen and the Haemovores. It points to how well organised the production was by 1972 that even a potentially mundane middle episode has so many entertaining moments.

Next episode: The Sea Devils – Episode Four