An experimental submarine, the “Siren II”, with a very experienced crew is sent to find out what happened to the “Siren I” after it mysteriously dissapeared in a submarine rift. Things go awry when they begin to find things that shouldn’t be there…

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.