At a small East Coast beach town, a boat dumps radioactive toxic waste into the ocean, which lands on a sunken ship and the skeletons of shipwrecked dead sailors. The skeletons and water plants are mutated by the waste, transforming into half-human, half-fish monsters that make their way to the surface.
Hank Green, an employee of scientist Dr. Gavin, attends a beach party with his immature girlfriend, Tina. Tina is drinking alcohol, and, when Hank disapproves, they argue. The party catches the attention of a motorcycle gang. Tina flirts with the leader, Mike, while Hank talks with Dr. Gavin's young-adult daughter Elaine. When Hank tries to take Tina away, Mike fights him. Hank wins, and Tina apologizes to Hank, but he leaves. Mike is taken home by his gang. Tina swims to some rocks far away from the beach, where one of the monsters kills her. The sight of her body causes commotion on the beach.
After Tina's death, Elaine worries about Hank but is reassured by her father. Eulabelle, Dr. Gavin's housekeeper, suggests that some kind of voodoo is responsible for Tina's death. Gavin refuses to believe this. Later that night, Elaine cancels going to a slumber party with her friends. The monsters, whose numbers have increased, attack the slumber party and kill most of the girls. This news spreads quickly.
Three female travelers drive through the town on their way to New York. Their car's tire goes flat near Fingel's Quarry. While attempting to fix the tire, they are killed by the monsters. One of the monsters, stalking two girls in town, is enraged when they are picked up before it can kill them. The monster, noticing a clothing store with female mannequins on display, breaks the window with a punch, ripping off its arm. Dr. Gavin and Hank study the severed arm, which is still alive, and they cannot figure out how to kill it. Eulabelle accidentally spills a container of metallic sodium on the arm, which kills it, giving the scientists a way of destroying the monsters.
Dr. Gavin and Hank search for the monsters. After discovering that the monsters can be tracked by their trail of radioactive water, Hank drives to New York City to obtain sodium. Dr. Gavin initially has no luck in finding the monsters. Eulabelle tells him that Elaine went to look for the monsters at Fingel's Quarry. Dr. Gavin rushes off to help Elaine, bringing a small case of sodium with him. Elaine tests the water for radioactivity, and the monsters chase her. She trips between two rocks and injures her leg. Dr. Gavin arrives and throws sodium at the monster attacking Elaine, killing it. Another monster attacks, but Dr. Gavin is out of sodium and must defend his daughter himself. Hank, who is bringing more sodium, is stopped by a police officer, who leads him to Fingel's Quarry. Hank saves Dr. Gavin by killing the monster attacking him, and the group kills the remaining monsters.
Hank visits Elaine, who is recovering from her leg injury.John Scott as Hank Green
Alice Lyon as Elaine Gavin
Allan Laurel as Dr. Gavin
Eulabelle Moore as Eulabelle
Marilyn Clarke as Tina
Agustin Mayor as Mike
Damon Kebroyd as Lt. Wells
Munroe Wade as TV Announcer (credited as Monroe Wade)
Carol Grubman as Girl in Car
Dina Harris as Girl in Car
Emily Laurel as Girl in Car
Sharon Murphy as 1st Girl
Diane Prizio as 2nd Girl
The Del-Aires as Vocal Group
Charter Oaks M.C. as Motorcycle Gang
(Uncredited cast)Robin Boston Barron as Biker
John Becker as Del-Aires Member
Gary Robert Jones as Del-Aires Member
Ronnie Linares as Del-Aires Member
Bob Osborne as Del-Aires Member
Floyd Keeler as Biker
Del Tenney as Gas Station Attendant
Wayne Tippit as Drunk Killed by Monster
Daniel Walker as Drunk #1
The film's working title was Invasion of the Zombies. Unlike the beach party movies filmed up to that time, this film was shot in black and white and on the Atlantic coast. Produced in Stamford, Connecticut, the beach scenes were filmed in an area of town called Shippan Point, and shot over a period of three weeks.
The biker gang in the film was played by the Charter Oak Motorcycle Club of Riverside, Connecticut.
The monsters for the film were constructed at Gutzon Borglum's sculpting studio in Stamford. There were two monster suits, and when they dried, one was too small for the stuntman. Production assistant Ruth Glassenberg Freedman had a son, Charles Freedman, who was 16 at the time. He fit perfectly into the suit and thus portrayed a monster in the film.
The "underwater" transformation scene of the monsters was actually shot on a stage with images of fish in an aquarium superimposed over the dissolving stage shots. Chocolate syrup was used for blood during the monster attack scenes.
The estimated budget of the film was $50,000.
The original theatrical release of the film paired it with another Del Tenney feature, The Curse of the Living Corpse, in a studio-sanctioned double feature.
The advertising for the double feature of Horror of Party Beach and Living Corpse capitalized on a gimmick first utilized by director William Castle, in which some newspaper advertisements included a call-out that stated: "For your protection! We will not permit you to see these shockers unless you agree to release the theater of all responsibility for death by fright!" Theaters were encouraged by the distributor to have patrons sign a "Fright Release" before they took their seats. The trailer for the double feature also included this claim.
Although billed in its promotional material as "The First Horror Monster Musical," all the songs heard in the film were presented as either soundtrack music or source music, as opposed to the style of a traditional musical with songs sung by central characters of the story. Ray Dennis Steckler's The Incredibly Strange Creatures Who Stopped Living and Became Mixed-Up Zombies also made the same claim a few months earlier.
As a tie-in, a monographic fumetti comic book by Wally Wood and Russ Jones detailing the film's story was released by the Warren Publishing Company under its Famous Films masthead. It had a 35-cent cover price.
Edward Earle Marsh (aka Zebedy Colt) composed the film's soundtrack; Wilfred Holcombe was credited as the musical director. Marsh and Holcombe wrote three songs that were performed in the film: "Joy Ride", "The Zombie Stomp" and "You Are Not a Summer Love."
Gary Robert Jones and Ronnie Linares, both of The Del-Aires (a Paterson, New Jersey rock band who play themselves as a local band), wrote one song together, "Drag," and one song each: "Wigglin' Wobblin'" (Jones) and "Elaine" (Linares). The Del-Aires performed all six songs in the film.
The Horror of Party Beach was included as one of the choices in the 1978 book The Fifty Worst Films of All Time.
Movie-mocking television series Mystery Science Theater 3000 featured The Horror of Party Beach in one of its season 8 episodes in 1997.
Punk band Sloppy Seconds's song "The Horror of Party Beach", appearing on their 1989 album Destroyed, detailed the plot of the movie.