[[caption-width-right:350:[[WesternAnimation/SpongebobSquarepants Down, down, down, to the bottom of the sea...]]]]
-> ''Nossing in ze vorld can ztop me now!''
-->-- What many fans think '''Zaroff''' said.

-> ''Nothing in the world can stop me now!''
-->-- What '''Zaroff''' really said.

The Doctor, with Polly, Ben and Jamie in tow, lands the TARDIS on a volcanic island where they are captured and taken into a cave complex. It soon becomes clear that this tiny island is all that remains of the ancient Atlantean civilisation.

Mad scientist Professor Zaroff (played with a LargeHam by Joseph Furst) has convinced the Atlanteans that he can raise the city from the sea, but in reality he plans to drain the ocean into the Earth's core and thereby destroy the planet. [[ForTheEvulz Just because]], OK?

Polly is taken off to be converted into an aquatic FishPerson, while the others meet two shipwrecked sailors who help them convince the Fish People to rebel. Eventually, the Doctor manages to foil Zaroff's plan by flooding the city before he can complete the drilling. Zaroff drowns, but everyone else makes it out and the Doctor and companions depart.

For a long time it was thought that [[http://www.dailymotion.com/playlist/xvbzd_tardismedia_the-underwater-menace only episode 3]] remained, until [[http://www.scifinow.co.uk/news/doctor-who-galaxy-4-and-the-underwater-menace-missing-episodes-found/ episode 2]] turned up in 2011.

* ActuallyPrettyFunny: Zaroff is only very briefly angry when he learns that the Doctor tricked him into saving his life with a [[BlatantLies blatant lie]].
* AllThereInTheManual: In an early draft of the story, and in the novelization, Zaroff's motivation to blow up the Earth stemmed from his wife and children dying in a car crash and driving him mad.
* {{Atlantis}}: Or at least the last remnants of that civilization, living underground.
* AuthorFilibuster: Against organized religion in part 4.
* BrokenAesop: As noted, the AuthorFilibuster has the Atlanteans blame [[ReligionIsWrong their religion for all their problems.]] Even though the actual culprit Zaroff didn't seem to have much of anything to do with their religion. Or that one of the few Atlanteans to see through Zaroff's deceptions and distrust him (even before the Doctor's help) was one of the priests of their religion. And the character making the Filibuster had been an antagonist who hadn't done anything good, sympathetic, or redemptive up to this point. The Filibuster was apparently supposed to be that character's redemption.
** Also, the prophecy did come true in a way.
* EarthShatteringKaboom: What Zaroff hopes to create.
* EvilIsHammy: Zaroff could have a HamToHamCombat with the Daleks and have a good chance of winning.
* FishPeople: Created by surgery rather than a natural species. Polly almost becomes one in some of the few clips to survive from episode 1 and in the newly-recovered episode 2.
* [[ForScience For SCIENCE!]]: The closest Zaroff comes to an explanation of his motives is pretty much this.
* HumanSacrifice: The Atlanteans try to sacrifice the Doctor and company to the goddess Ando... using sharks.
* ICantBelieveImSayingThis:
-->'''Jamie [=McCrimmon=]:''' You know I never thought I'd say this, but well it's great.
* LaResistance
* MadScientist: Zaroff is a classic example.
** At the end of the Episode 2 clip on the BBC site, he sounds like a proto-Davros, on how succeeding with his [[OmnicidalManiac omnicidal mania]] would be the ultimate power.
* {{Mythpunk}}
* OminousLatinChanting: multiple times in the temple of Amdo.
* ReligionIsWrong: The conclusion the surviving Atlanteans reach through [[BrokenAesop dubious]] [[AuthorFilibuster means]] at the end.
* SuicidalCosmicTemperTantrum: Zaroff.
* WhatTheHellIsThatAccent: [[RealityIsUnrealistic It's Joseph Furst's real one, that's what]].