On a sunny California beach, three surfers and their girlfriends enjoy a carefree day of fun and sun. Among them is Bunny, who is playfully running away from her boyfriend. Later that day, tragedy strikes when Bunny is found dead near a cave, her body attacked by a strange, humanoid creature covered in seaweed. The local sheriff is quick to investigate, discovering mysterious footprints in the sand, which he carefully casts and sends to Dr. Otto Lindsay, a renowned oceanographer, for analysis.
Dr. Lindsay, played by Jon Hall, examines the footprints and suggests that they could belong to a "fantigua fish"—a large, carnivorous marine creature that has mutated into a terrifying hybrid, capable of surviving out of water in a grotesque, seaweed-shrouded, anthropomorphic form. The proximity of the Santa Monica beachfront to Dr. Lindsay's house makes it easy for suspicion to grow around the nearby water and sand.
In his household, Dr. Lindsay is accompanied by his son, Richard Lindsay, portrayed by Walker Edmiston, who used to assist his father with research on mutations but now prefers a more adventurous, hedonistic lifestyle. Richard's stepmother, Vicky, played by Sue Casey, is a restless woman who feels neglected by her older husband. She tends to drink, flirt, and seek excitement, constantly craving more out of life. Richard also has a close friend, Mark, a sculptor with a limp resulting from a car accident caused by Richard. Mark, played by Walker Edmiston, lives in a guest house attached to the main house and often offers Richard advice about life and relationships.
The story unfolds with scenes of surfing in Hawaii, showing the carefree spirit of the characters, but lurking beneath is the darker element—the creature. Richard, tired of the research world and eager to live fully, chooses to avoid the beach and instead spends time swimming in the pool with his girlfriend, Jane, played by Elaine DuPont. Meanwhile, Vicky ventures onto the beach alone, unknowingly stumbling close to the creature's hunting ground. Her daring attempt to bask on the sand almost leads to her becoming the beast's next victim, but she returns home unharmed at first.
The tension escalates when Dr. Lindsay tries to persuade Richard to rejoin the scientific effort to understand and possibly eliminate the menace, but Richard shakes his head, more interested in partying and living for the moment. That evening, a beach party full of dancing and music becomes the scene of violence when the creature attacks a surfer named Tom, leaving him horribly slashed and dead. Mark witnesses the attack but is unable to stop it, only finding a strange strip of rubber in the sand—evidence pointing to the creature's presence.
The night takes a darker turn when a heavily intoxicated Vicky returns home, only to be lured by a typed note supposedly from Mark, asking her to meet in his room. There, she is confronted and attacked by the creature, which leaves her with a grotesquely scarred face. Mark arrives just in time to find her in distress and, during a struggle, discovers a rubber mask that matches the creature's appearance. In a desperate attempt to defend himself, Mark stabs at the creature, tearing off its mask to reveal a shocking reality: the creature is actually Dr. Otto Lindsay, the lead oceanographer and father of Richard, who has apparently transformed into this monstrous being.
In the chaos that ensues, Dr. Lindsay escapes in Jane’s car, driving away with a wild desperation. Richard and Jane, shocked and grieving, find Mark gravely wounded and dying from his injuries. They contact the police, and a high-speed car chase ensues as they pursue Dr. Lindsay, who seems determined to continue his deadly work. The pursuit ends tragically when the car crashes and bursts into flames, claiming Dr. Lindsay's life. The sheriff suggests that Dr. Lindsay's actions were driven by a twisted obsession, possibly for the sake of his son Richard, who was clearly the unintended victim of his scientific curiosity.
This story is a haunting tale of science gone wrong, the terrifying consequences of mutation, and a family's tragic descent into chaos, all set against the backdrop of a seemingly idyllic beachside community.
