It’s all go in this final episode, which is a vast improvement over last week’s misfire. Predictably, the Sea Devils prove as untrustworthy associates to the Master as the Nestenes, the Mind Parasite and the Axons, and he ends up locked in a cell with the Doctor who, fortunately, has reversed the polarity of the neutron flow (yay!) to destroy the Sea Devils and their base before they can take over the world.

I like how much this prioritises dynamic action rather than re-heating the finale of Doctor Who and the Silurians (the Doctor fiddles with a reactor then sees some explosions from a distance). The Sea Devil attack on HMS Seaspite looks like Douglas Camfield could have staged it. Michael E. Briant makes the greater than usual number of Sea Devil costumes work for their money, so this is as close to an army of monsters as we’ve ever seen in the series. While this is going on, Katy Manning gets to climb around buildings and drive a hovercraft, Jon Pertwee gets to ride on a Jet-Ski, and at the end, when it could easily have been two stuntmen bobbing around in the water waiting to be rescued Briant makes sure we see it really is Pertwee and Delgado. It’s only a small thing, but it’s a brilliant touch at selling the episode.

It’s traditional to compare this to Doctor Who and the Silurians. That feels futile, though. This is not a remake, it’s a companion piece in a different, action genre, a long way from the moralistic Quatermass sci-fi horror of Season Seven. There are obvious plot similarities, but this never boils down to an ethical stand off between two parties for ownership of the Earth. The Sea Devils are more straightforward monsters than the Silurians. The Doctor gives them every opportunity to choose peace, but they declare, ‘We shall destroy man and reclaim the planet.’ In response, the Doctor makes exactly the same choice the Brigadier made in the earlier story: he blows them up. No second chances: that’s the kind of man he is.

Importantly, unlike in the earlier story, this isn’t presented as a morally dubious or grey decision: ‘I did what I had to to prevent a war.’ And although the British government is planning a nuclear strike the script is clear that the Sea Devils were destroyed before the attack could take place. It was the Doctor’s work. If you like, you could see this as his morals being diluted from his association with UNIT, but there’s nothing in the actual text to support this reading.

The only ambiguous part of the ending comes a few moments later, when the Master escapes his exile, and Pertwee’s expression is inscrutable – perhaps even the flicker of a smile. Pertwee and Delgado’s friendship has bled into the characters of the Doctor and Master, making what could have been a fairly vanilla relationship look a lot more interesting. They say they’re enemies, but almost every interaction they have suggests that neither actually wants to lose the other. The Master steps in to prevent the Sea Devils from killing the Doctor, who repays the favour by helping the Master to escape the doomed undersea base.

I adore this story, burbling music and all. Along with The Dæmons, Planet of the Daleks and The Green Death this was repeated on BBC2 in the early 1990s, and is one of the few Doctor Who stories I can remember making much impression at school. As such, Jon Pertwee and the Sea Devils are as much a part of my childhood as Davison or McCoy, and generate the same buzz of nostalgia as the Kandy Man or the Tetraps. And after Day of the Daleks and The Curse of Peladon that’s three very good serials in a row. The show is on a roll!

Next episode: The Mutants