A radiation leak at an atomic power plant causes a bizarre mutation in the sea life off the California coast. The mutation evolves into the form which best allows it to exist in its environment - an amphibian like creature which can forage for food both in the sea and on the land. As the creature eats its way out of its spawning grounds it moves up the coast and into the channel of the world's largest man made small craft harbor, The Marina Del Rey. From the Marina channel the creature works its way into the Grand Canal of Venice, California and new source of nourishment becomes available - water fowl, dogs, cats and PEOPLE!

Wayne Connors, a frustrated high school journalism instructor whose ambition is to become a reporter, reads about the mysterious disappearance of house pets around the canal area of Venice. When several animals are found mutilated, the local residents are concerned and when a middle aged couple is murdered in what appears to be a Manson type slaughter, the area erupts in panic.

Dissatisfied with the ritualistic cult murder theory held by the police and sensing that the story could make him an instant reporter's reputation, Wayne Connors sets out to unravel the mystery of the bizarre murders. The story leads Wayne, his girlfriend Jeff, his sidekick Dr. John (a high school zoology instructor) and a black Jamaican diver named Christopher Columbus Alexander, on an adventure seldom experienced. From the opulence and beauty of the Marina Del Rey to the seedy transient life of old Venice, to an isolated dome house high in the Malibu mountains and back out to the Pacific Ocean, Wayne and his friends track down the mystery that is "the Slithis".