[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/fish_people_151.jpg]]
-> [[BeamMeUpScotty ''Nossing in ze vorld can ztop me now!'']]
-->-- What many fans think '''Zaroff''' said.

-> [[NothingCanStopUsNow ''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.

!!Tropes
* 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, Zaroff's motivation to blow up the Earth stemmed from his wife and children dying in a car crash.
* {{Atlantis}}: Or at least the last remnants of that civilization, living underground.
* AuthorFilibuster: Against organized religion in part 4.
* ClicheStorm: To the point that the recovery of another episode was met with decidedly more mixed reactions that literally any other story would have gotten. Opinion has shifted as fans have seen the episode. It's generally considered much better than episode 3, and improves episode 3 by adding much-needed context.
* EarthShatteringKaboom: What Zaroff hopes to create.
* 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.
* 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.
* MissingEpisode: Only parts 2 and 3 survive.
* MythPunk
* LaResistance
* SuicidalCosmicTemperTantrum: Zaroff.
* WhatTheHellIsThatAccent: [[RealityIsUnrealistic It's Joseph Furst's real one accent, that's what]].