Near a quiet East Coast beach town, a boat dumps a 55-gallon drum labeled Danger Radioactive Waste into the ocean. The drum sinks to the seabed, ruptures beside a sunken ship and a nearby skeleton, and its contents awaken a creature that mutates into an aquatic, humanoid monster. The monster makes its way toward the shore just as a beach party, with The Del-Aires providing the soundtrack, fills the air with music and laughter.
Hank Green, [John Scott], an employee of local scientist Dr. Gavin, [Allan Laurel], arrives at the party with Tina, [Marilyn Clarke], his girlfriend. They clash in a tense exchange, Tina flirts with Mike, [Agustin Mayor], the motorcycle gang leader, while Hank talks with Elaine Gavin, [Alice Lyon], Dr. Gavin's daughter. A confrontation erupts between Hank and Mike, and Tina, after being spurned, swims toward a rock jetty where the monster attacks and kills her. Her body washes ashore, stained with blood, a grim first sign that something monstrous has slipped into their seaside town.
The police enlist Dr. Gavin to help with the investigation, and he proposes using Carbon-14 tests to study a tissue sample and glean clues about its origins. Eulabelle, [Eulabelle Moore], the Gavin household’s loyal housekeeper, mutters that “voodoo” might be to blame, offering a superstition-filled counterpoint to the scientific approach. Elaine confesses her romantic feelings for Hank, and she later decides to skip a slumber party with her friends. The killer creature multiplies, and the monsters—likely the same radioactive transformation triggering extra attackers—crash the slumber party, killing more than twenty attendees and turning a neighborhood sleepover into a scene of panic and fear.
Later, three female travelers stop to change a flat tire and are swiftly slain by the advancing monsters. As Hank and Elaine attend an evening dance on the beach, one monster stalks two young women wandering through town. In a chilling display of its reach, the creature attacks female mannequins in a storefront window, tearing off its own arm in the process. Dr. Gavin and Hank study the severed limb, with Gavin describing the muscle tissue as “a sea anemone, a kind of protozoa.” Though they cannot yet devise a method to kill the creatures, a stroke of luck arrives when Eulabelle accidentally spills a container of metallic sodium on the arm, destroying it in a fiery blaze. Gavin realizes a chemical method for destroying the monsters: plain old sodium.
Two drunken men stumbling through town stumble upon a dead, mutilated man inside a parked truck, and one of the drunks is quickly attacked and killed by the monsters. A montage follows, showing additional women being attacked and killed while the police chase the elusive trail of radioactive water left by the creatures. Hank leaves for New York City to acquire a larger supply of metallic sodium, and Elaine, determined to confront the threat, searches a local quarry where the earlier victims were found. When she discovers the high-radioactivity waters, she also spies the emergence of a monster. She manages to avoid being seized, but a leg injury slows her escape. Dr. Gavin rushes to her side and, by tossing sodium, destroys an approaching monster in a fiery explosion. As more monsters close in, Gavin engages one in extended hand-to-hand combat. Hank returns with a larger stock of sodium, kills the monster attacking Gavin (in the process badly burning Gavin), and, with the aid of the police, they destroy the remaining monsters and end the immediate danger to the town.
