In 1914, Milo Thatch, grandson of the great Thaddeus Thatch, works in the boiler room of a museum. He knows that Atlantis was real and that he can get there if he has the mysterious Shephard's Journal, which can guide him to Atlantis, but he needs someone to fund a voyage. His employer thinks he's dotty and refuses to fund any crazy idea. He returns home to his apartment and finds a woman there. She takes him to Preston B. Whitmore, an old friend of his grandfather. Whitmore gives Milo the Shepherd's Journal, a submarine, and an all-star crew. They travel through the Atlantic Ocean, face a large lobster called the Leviathan, and finally get to Atlantis. But does the motivations of the Atlantis crew are revealed to be different than Milo's.