A young European boy living in San Francisco is reluctant to marry his long-term girlfriend because he wants to travel around the world first. His wealthy uncle agrees to send him on a global expedition aboard his ship, but en route the boy and his travelling companion are shipwrecked on a remote island, populated by countless prehistoric creatures as well as gold-hunting bandits.

People are dying mysteriously and gruesomely, and nobody has a clue what the cause is. Only health worker Mike Brady has a possible solution, but his theory of killer slugs is laughed at by the authorities. Only when the body count begins to rise and a slug expert from England begins snooping around does it begin to look like Mike had the right idea after all.