Tenth entry in the Carry On series. Able seaman Poop-Decker (Bernard Cribbins) signs up for adventure on the high seas with the wicked Captain Fearless (Kenneth Williams). Those swabbing the decks include Juliet Mills, Charles Hawtrey and Donald Houston. The film was originally to be entitled ´Up the Armada´, but the British Board of Film Censors objected to such a rude title.

Searching for the lost world of Atlantis, Prof. Aitken, his son Charles and Greg Collinson are betrayed by the crew of their expedition’s ship, attracted by the fabulous treasures of Atlantis. The diving bell disabled, a deep sea monster attacks the boat. They are all dragged to the bottom of the sea where they meet the inhabitants of the lost continent, an advanced alien race that makes sailors their slaves.