The monster, which looks like a snarling "Creature from the Black Lagoon," invades a sleepy seaside town. The lighthouse keeper, newly widowed and estranged from the town folk, has been leaving food out for the monster for years, unaware of it's blood lust. When the monster's appetite outstrips the keeper's ability to serve it, bloodless decapitated corpses start to show up.
A small coastal town is haunted by a spree of horrific murders. The victims include a talkative shopkeeper, a little girl, two brothers out fishing, and a sheriff's deputy. A pair of scientists and the sheriff are baffled at the cause of these slayings until they run into a Creature of the Black Lagoon style monster. The monster is a missing link between water-breathing and land-breathing creatures and is prehistoric. It turns out that the grumpy lighthouse keeper has been feeding the monster for the last ten years. After a final showdown between the creature and the lighthouse keeper, both are killed. One at the hands of the monster and the other by being pushed off the top of the lighthouse after being disoriented by the bright light.
- The movie opens with a long shot of a lighthouse along the California coast. A reptilian claw comes up from behind a rock outcropping and grabs a metal bowl on a chain. The keeper, Sturges (John Harmon) exits the lighthouse, checks his watch, and prepares to mount his bike for a ride into town. He notices some kids walking along the cliffs and shoos them away, warning them not to come back. The credits roll.
A group of people are gathered around a small fishing boat on the beach, gawking. One man observes, "Heads ripped clean off." Constable George Matson (Forrest Lewis) notes that the bodies look like they have no blood. Sturges bikes to Kochek's Store. He gives Kochek (Frank Arvidson), the storekeeper, his list of supplies. Kochek asks Sturges about the Rinaldi Brothers, the two bodies found on the beach. Kochek claims he found them. When Sturges asks about his meat scraps, he is told they were sold to someone else. As they finish business, two men bring the corpses of the Rinaldi Brothers into the store in a wheelbarrow. Kochek puts them in the refrigerator to keep until an autopsy can be completed. Sturges walks his bike down the road to the Wings Café. He goes in to see his daughter, Lucille (Jeanne Carmen) who works behind the counter. Constable Matson is the owner, and Lucy tells her father she has to work late. Sturges is worried about her return home in the dark. He has good reason to be concerned, as he is harboring a grim secret. Lucy assures her Dad she'll be fine, Fred (Don Sullivan) will bring her home safely in his Jeep. Fred is a biology student and talks Lucy into joining him later collecting specimens from the ocean. Constable Matson asks to speak to Sturges before he leaves. He questions him about the Rinaldi Brothers and whether he saw them and knows anything about the circumstances of their deaths. Sturges lies and says he doesn't know anything.
Dr. Sam Jorgenson (Les Tremayne) finishes his exam of the Rinaldi Brothers at Kochek's store. He confirms that their heads were cleanly and expertly severed--Death was instantaneous. Kochek tells them of the legend of the Monster of Piedras Blancas. Matson warns Kochek to keep his stories to himself; the town's upset enough already.
Fred and Lucy arrive at the beach. She spreads a blanket and sets out lunch. They talk about the investigation. Sturges chains a metal dish to the rocks and tosses a few fish out as bait. He then leaves the cliffs. Matson and Dr. Jorgenson meet at the café. He tells the constable there was one final matter about the autopsy. He says the main artery was such that something pumped out all the blood from the victims. Fred brings Lucy back to work in his Jeep. Matson asks Fred to come in for a talk. That evening Fred brings Lucy home. Sturges doesn't like Fred. She tells Fred she's been away at boarding school for ten years and only recently started coming home for summer vacations. Fred drives off and Lucy decides to go for a swim, instead of going directly inside. She leaves her clothes on the rocks. We see the arm and claw of the creature as it paws her clothes. Her father calls her; she returns from the surf, dresses and goes inside. When she mentions that she had the strangest sensation she was being watched, her father gives her a stern lecture about swimming at night and threatens to ship her back to boarding school. She has no idea her dad has been feeding the creature.
At Kochek's store that night the creature enters and kills the shop owner. The next morning the Rinaldi Brothers funeral is underway. The two caskets are brought from the church. A mother tells her crippled son Jimmy (Wayne Berwick) that he doesn't have to come to the cemetery for the graveside service. He picks up a piece of wood and begins whittling. He spots a quarter on the street and retrieves it. He takes his new found wealth into the store to buy candy and calls Mr. Kochek. He wanders around when he gets no reply. Then in the office area he spots the headless corpse and runs from the store straight to the cemetery. He tells Dr. Jorgenson, who is officiating at the service, that Kochek was murdered. They assemble at the store and Matson confirms the death. He asks the Doctor to examine the shop keeper. He tells Eddie (Pete Dunn) to go outside. Eddie is visibly upset and perhaps nauseous. Fred arrives at the store. They look around and try to establish time of death--they estimate 2:00 a.m. Dr. Jorgenson calls Fred over to examine something. He thinks it might be a fish scale, but too large. They decide to return to the doctor's office to perform some tests. Matson tells Eddie to come back in and move the body to the ice room then lock up and go home.
At the doctor's home, Jorgenson and Fred examine the scale. Matson is there and is getting impatient and voices his frustration at how long the tests are taking. They tell Matson the scale is similar to an extinct amphibious reptile. Lucy runs to the doctor's house calling for Matson. She found her father at the bottom of the cliffs near a cave and he is unconscious. They pile into Fred's Jeep and race to the lighthouse. Jorgenson, Matson and Fred revive Sturges and move him back to the lighthouse. They set him on the couch. Lucy notices the dog, Ring, is missing. Sturges and his dog are inseparable. Fred tells Matson and Jorgenson to take his Jeep, he will stay with Lucy. Fred tells Sturges that Kochek was murdered and asks about the legend.
Matson and the doctor arrive back in town and see a man carrying the body of a little girl followed by a crowd of townsfolk. He places the girl's body on a table in the café. The man is in shock, but apparently the little girl, his daughter, was also a victim of the monster. He does manage to relay that her mother sent her to the store. The doctor and Matson leave to go check with Eddie. They left him at the store after Kochek's death.
Fred and Lucy discuss the circumstances of her ten year absence from town at boarding school. Fred insists on searching the caves near the lighthouse. Lucy objects to this, because her father has strict orders that no one is to go in or near those caves. They argue, and she tells Fred he shouldn't come back to the lighthouse again. Matson and a few of the town's people call for Eddie at the store. A small crowd has gathered outside. Matson goes into the ice room. We hear a roar, and Matson screams. He exits wounded, followed by the monster (Pete Dunn in a second role) carrying the head of Eddie in his right claw. One member of the crowd takes a swing at the monster with a meat cleaver. They find a scale on the cleaver later. Matson proclaims, "I guess we've found our killer."
Matson takes the Jeep to get Fred. Fred is examining the rocks and finds the chain, but the dish is missing. Matson warns Lucy to lock herself inside. There have been two more murders. Matson finds Fred and tells him about the monster. Fred and Matson leave in the Jeep. Lucy looks in on her father. They arrive at the café and Matson gives Fred a rifle and he grabs a pistol from behind the counter. They form a six man posse and search for the creature. They spot his foot prints in the sand, enter a cave and find another severed head. Two of the posse have been attacked and injured. They decide to regroup the next day with more men.
Sturges finally tells Lucy that many years ago he explored a cave and walked many miles to an opening near the lighthouse. He heard something like heavy breathing. The next day he left some fish and the day after they were gone. He left fish daily and then got meat scraps from Kochek to feed the creature. Lucy finally concludes that her father had been feeding the creature all these years. That's why he sent her to boarding school--for her own safety. Lucy helps her dad up to tend the beacon. He tells her to bolt the door downstairs. Lucy goes out to feed her dog. She leaves his food dish outside the door. She doesn't realize Ring is dead.
The doctor, Matson, and Fred devise a plan to capture the creature using a net. The creature comes to the lighthouse and forces the front door. He enters and heads for the stairs to the light. Lucy calls out thinking it is her father. The creature turns around. Lucy opens her bedroom door and is startled to see the creature and screams. Sturges hears the screams of his daughter.
Matson returns to the Garage and finds the doctor, Fred and the owner working on the net. Matson tells Fred to call Lucy; he did not see the lighthouse light on and is concerned. Fred telephones and gets no answer. Fred and Jorgenson take the Jeep to the lighthouse; Matson forms a posse and meets them later.
The monster carries Lucy outside. Sturges see this from the top of the lighthouse. He pitches a lantern at the creature and strikes it on the head. The creature puts Lucy down and walks back to the lighthouse to deal with his tormentor. Sturges meets it on the staircase and fires his rifle into the creature with little effect. It chases Sturges back up the spiral staircase to the light. Lucy runs for help and encounters Fred and the doctor. Sturges gets outside via a steel door and bolts it from the outside. The posse and Fred, Lucy and the doctor arrive at the lighthouse. The creature bangs on the steel door. It forces the door and chases Sturges up a ladder on top of the light. It picks Sturges up and tosses him off the lighthouse. Fred has gotten up in the lighthouse and outside and confronts the monster with his rifle. He notices a flashlight beam from below seems to distract the creature. He orders Lucy to turn on the beacon. The light blinds the creature and as it reels back against the railing, Fred pushes it over with the butt end of his rifle. It crashes into the water below. Fred and Lucy embrace, and we close with a shot of the lighthouse.