When a young girl named Bunny is killed on the beach, the police theorize that it is either a psychopath or something more sinister. However, oceanographer, Doctor Otto Lindsay, claims that it is a giant humanoid like fish that was responsible. Lindsay also uses the occasion to rail against the local surfers that his son Rick has been hanging around with since he was involved in a near fatal accident that also left Rick's friend Mark a cripple. Soon, there are other killings, including one of Rick's surfing buddies, Rick,s adulterous step-mother Vicky and Rick's crippled friend Mark. However, during the attack on Mark, the monster is cornered and after a struggle its mask is ripped off and the monster is revealed to be Otto, who was committing the murders as a twisted way of trying to make Rick give up his friends and to get back at Vicky for her affairs. Otto then steal a car belonging to Rick's fiancee Jane and after a short chase he drives off a cliff and dies in a fiery crash.

Richard has not become an oceanographer as his father (a professor at the Oceanic Research Laboratory) has wished, and he spends his time surfing with his friends. He and his fiancée, Jane, find the mutilated body of a girl on the beach, and later, at a beach party, Mark, a loner, discovers that his friend Tom also has been murdered. The evidence points to a sea monster, and Mark is suspected of disguising himself and committing the crime. He begins to investigate and finds footprints that lead him to Richard's house, where Richard's young and attractive stepmother, Vicki, tries to seduce him. The monster suddenly enters the room and kills Vicki, but in its struggle with Mark is unmasked to reveal Otto, Richard's father. As the police approach, Otto escapes by automobile, but after a chase through Hollywood Hills, he crashes and dies.