A newly unveiled ocean liner, the Argonautica sets out on its first cruise, only to encounter dangers both on board and in the ocean depths. While the beautiful thief Trillian (Famke Janssen) attempts to steal riches from the boat's wealthy passengers, the ship encounters major problems, most notably a giant murderous sea creature with tentacles. Eventually, a boat captained by John Finnegan (Treat Williams) comes across the Argonautica, and everyone struggles to survive the monster.