Young archaeologist Dr. Valerie Cammon (Polly Shannon) and her colleague Dr. Kim (Philip Moon) go by ship to a volcanic island in the Mediterranean sea. While she seeks relics in a cave, a seaquake sinks their ship without survivors. At about the same time all archaeologists except for Dr. Cammon get eaten by the re-awakened ancient Lernaean Hydra.
During the tempest, a whole island disappears. Two months later, Vincent (Alex McArthur) and Dixie Camden (Jana Williams) intended to organize a man-hunt for cranky millionaires. Their Captain (Michael Shamus Wiles), unaware of the Hydra, suggests the nearby, aforementioned island as a substitute.
Four ex-convicts, Tim Nolan (George Stults), Gwen Russell (Dawn Olivieri), Bob Crick (James Wlcek) and Ronnie Kaplan (Texas Battle), are marooned at the shore and given 24 hours' headstart.
The following day the rich hunters (played by Paul Rae, Roark Critchlow, Ricco Ross and William Gregory Lee) arrive, but attract the Hydra's attention and get eaten. The same fate befalls some of the fugitives.
Dr. Cammon runs into the surviving fugitives and helps Tim to find the magic sword of Herakles. As Dr. Cammon tries to distract the Hydra, Tim gets the sword so he can now fight the beast, but, sadly, he retrieves the sword too late and Dr. Cammon is torn apart and eaten by the Hydra. Tim manages to cut off its heads for good, but fails to cut the last head completely off. With Dr. Cammon dead, the remaining fugitives are forced to go on without her help.
Eventually, the only ones left are Tim and Gwen and they return to the ship,not knowing that the Hydra has recovered and is following them. Now only one-headed, it can move like a snake and makes its way into the belly of the ship.
Gwen wants to use the radio for calling SOS but the microphone is damaged. Mr. Camden blindsides her and blackmails Tim. The Hydra kills Camden from behind, then devours Dixie. Tim grabs the magic sword again and this time, he really kills the monster.
Tim and Gwen reach Sardinia and use the $10,000,000 to start over again as a couple.
