It turns out that there is a mad scientist under the sea in Atlantis barbarically turning people into fish-people so they are able to breathe in their new habitat. This man promises to raise Atlantis to the surface in order to blow up the world, for the sheer achievement of it; which is the most ridiculous set up for a story ever.

Then The Doctor gets dressed up... again! This time he looks like a cross between Bob Dylan and a member of Def Leopard. An insane chase begins between The Doctor and Zaroff, Troughton is clearly trying to differentiate himself from Hartnell's Doctor, he is more agile, less serious, more fun.

Perhaps the funniest moment of Part Three is the Irish guy trying to convince the fish-people to go on strike, and not getting very far at all, at least initially riling them up instead.

Zaroff kills the priest in another gruesome scene... it's odd it flits between comedy to horror. After yet another fight, Zaroff kills the emperor after being challenged by him, then we have the famous quote, "Nothing in the world can stop me now!!" Which actually comes across much better within the context of the episode than it is ever portrayed within fandom.

Part Four definitely feels like the end of an action film, the underwater base being flooded and an evil mastermind intent on completing his work would rather die trying than give up; our heroes have to get out of there before the place is completely flooded, or worse - blown up! It is fantastically realised (even without visuals).

There is an interesting anti-superstition element to the ending, no more temples, no more worshiping Goddesses; today this would be read as an anti-religious story, I wonder if that was in the mind of the writer.

No-one comments on the fact that a whole group of mutated humans have been left to fend for themselves in the water. I demand a Big Finish sequel!