In the damp confines of her cellar, Muffy St. John (Deborah Foreman](/actor/deborah-foreman)) fondly reflects on her 10th birthday celebration when she opened a jack-in-the-box that startled her with a tiny alien, bringing laughter from her guests. Simultaneously, a lively group of college friends await a ferry to transport them to Muffy's idyllic island cottage for the weekend. Among them are Kit (Amy Steel), her boyfriend Rob (Ken Olandt), and another couple, Nikki (Deborah Goodrich](/actor/deborah-goodrich)) and Chaz (Clayton Rohner). Rounding out the crew are Arch (Thomas F. Wilson), Nan ([Leah Pinsent), Hal (Jay Baker), and Muffy’s cousin Skip (Griffin O'Neal).
Onboard the ferry, Skip and Arch engage in a reckless game of 'stretch'—a twist of fate occurs when Arch throws the knife, inadvertently injuring Skip. The prank escalates when Skip, splashing into the water, draws alarm from the crew, leading to a chaotic scene where the crewman Buck (Mike Nomad) suffers injuries when struck by the ferry during the commotion. In the aftermath, consternation arises as Constable Porter (Tom Heaton) berates the group for their antics, instructing them to remain island-bound.
Upon reaching her home, Muffy warmly welcomes her friends, showcasing her stylish living and dining spaces. That evening, the group gathers for dinner, though Skip's mood remains subdued due to the earlier mishap with Buck. Muffy's playful tricks at the table bring comic relief, but they quickly segue into an unsettling series of events. As they settle into their respective rooms, Muffy's antics continue—trick cigars explode, water spurts unexpectedly, and ominous newspaper articles resurface, linking back to sinister incidents.
The next day unfolds ominously; Muffy’s behavior grows erratic as the friends discover Skip's body washed ashore and the group panics over his mysterious disappearance. Each person grapples with their own mounting anxieties: Arch is ensnared in a snare trap, while Hal and Nikki's outing to a well leads to a grim discovery—bodies hidden within. As fear mounts, Muffy's peculiar demeanor becomes ever more alarming.
Accompanied by dread, the group attempts to lock down the house as they await Constable Porter, who promises vital news. Muffy's inexplicable changes in appearance and behavior raise suspicions, culminating in a fierce argument between her and Nan about a past secret—a personal event that had remained buried. As chaos ensues and individuals go missing, the line between jest and grave danger blurs.
With desperation, Kit and Rob uncover terrifying evidence of the sinister happenings, leading them to connect the dots; Muffy may not be who she seems. In a nail-biting climax filled with tension and horror, Kit faces off against a lurking menace in the house, only to realize that the whole escapade is nothing more than an elaborate prank orchestrated by Muffy and her friends.
> "April Fools!" Muffy exclaims, as the façade shatters and everyone bursts into laughter, relieved but somewhat annoyed at the magnitude of the setup.
As the evening wears on and the group toasts to their newfound clarity, a lingering sense of unease remains. Muffy’s late-night return to her room reveals a troubling surprise; a jack-in-the-box sits ominously on her bed. As she tentatively cranks the handle, she is startled not just by the mechanical toy but by an unexpected attack from Nan, who unleashes a final, seemingly harmless prank, brandishing a fake knife with a joyful "April Fools!"—but the chilling conclusion leaves viewers questioning the line between jest and reality.
