When a young adventurer named Milo Thatch uncovers an old journal containing clues to the whereabouts of the sunken city of Atlantis, he wastes no time in putting together an expedition and setting off in search of the maritime metropolis. Heading deeper and deeper in their submarine, Milo and his crew brave many dangerous experiences, including a confrontation with a giant lobster, and are overjoyed when they finally find the underwater kingdom. However, after they reach their goal, it's not long before they discover a plot to seize all the power and riches it has to offer, and soon find themselves becoming the unlikely defenders of the lost civilisation.