There’s a lovely scene at the start of the episode where the TARDIS team contemplate where they might have arrived this time. Polly hopes it’s Chelsea 1966 – still desperate to get back to normality. Ben is sarcastic, Jamie is fearful and the Doctor is excited by the thought of ‘prehistoric monsters’. It’s a lovely little introduction to the new crew

With hindsight this episode does look like things are starting to settle down. The Doctor’s clothes have inexplicably regenerated into his familiar costume (no more massive checked trousers, and the stovepipe hat is stoved in), and Troughton’s performance doesn’t have quite the eccentric, manic pixie dream Time Lord feel it’s exuded so far. He’s still calling himself Dr W though (after The War Machines and The Highlanders it’s pretty obvious Lloyd and Davis think that really is his name). Confronted with guards and priests wearing outrageous fish hats, he doesn’t demand one like that. Instead, he’s the authoritative voice of exposition, explaining caisson disease, showing a familiarity with Cold War politics, and recognising the handiwork of Professor Zaroff).

It’s almost as if when he’s faced with Zaroff and his Atlantean guard, the maddest character so far, there’s no point the Doctor trying to put on an outrageous foreign accent or a silly costume. Instead, he treats Zaroff with absolute seriousness. Which has the effect of making the mad professor seem both even madder, and even more dangerous.

For all this story’s reputation, this episode is immediately engaging. The location filming (and volcano model) look impressive and atmospheric, and the introduction to the Atlanteans with their lionfish costumes and underwater temple is a lot more exotic than anything the show’s done for about a year. The established regulars each get a couple of fun lines – Ben telling Polly, ‘You speak foreign!’ and Polly deducing when they’ve arrived from a souvenir bracelet, and delivering the immortal line, ‘You’re not turning me into a fish!’ Frazer Hines, inserted at the last minute, does get a short straw with Jamie mainly asking the typically unrewarding ‘What’s that Doctor? Are you OK Polly?’ kind of questions. And with some really clear peril – man-eating sharks, a mad scientist, and fish-conversion therapy, this is really fun.

Next episode: The Underwater Menace – Episode 2