In the cellar of her house, Muffy St. John (Deborah Forman) picks up a jack-in-the-box, and thinks back to her 10th birthday party where she received it as a birthday gift (a tiny alien which jumped out and scared her, making the other party guests laugh).Meanwhile, a group of young college students wait at a dock to be picked up by a ferry and transported to Muffy's island cottage for the weekend. They include Kit (Amy Steel), and her boyfriend Rob (Ken Olandt), Nikki (Deborah Goodrich) and her boyfriend Chaz (Clayton Rohner), and Arch (Thomas F. Wilson) whom are best friends. The rest of the group includes Nan (Leah King Pinsent), Hal (Jay Baker), and Muffy's cousin Skip (Griffin ONeal). On the ferry, Skip and Arch play a game of 'stretch' (throwing a switchblade and both trying to stretch for it), until an irritated Arch throws the blade at Skip and hits him in the stomach. Skip falls off into the water in which the ferry crewman Buck (Mike Nomad) and Rob jump in only to see Skip with a trick knife-belt. Skip and Arch laugh at their April Fool's Day joke which happens to be that very day. Buck stays in the water and tries to hook the ferry as it comes into the dock, and as the group argues, Buck is hit by the ferry. He jumps out of the water with a chopped face. Buck is taken away by the ferryman (Lloyd Berry) in a motorboat screaming "they did it!" A local policeman, Constable Porter (Tom Heaton), is angry at the kids for letting this prank get out of control and tells them to stay put on the island as he accompanies Buck to the local hospital.Muffy takes the group up to her house where she shows them the living and dining room areas and shows them their quarters to spend the night. That evening, the group sits around the dinner table as Skip, still upset over what happened to Buck, stays outside and drinks. Muffy, with a smirk on her face, invites everyone for dinner as Nan sits on a whoopee cushion, and Arch leans back in a breakaway chair. They talk over what they will do after graduation in which Muffy tells them about her inheriting her father's house and talks about turning it into a country inn. They drink to a toast as Muffy's smirk gives away another gag she sets up as everyone spills champagne on themselves from the trick drippy glasses as Muffy laughs.They retire to their rooms for the night as they encounter more gags left behind my Muffy, which includes a trick cigar that blows up when Hal lights it; spurting water faucets, and trick lights that don't turn off. Hal finds snipped newspaper articles about people dying in a fog and a fire, which he apparently was involved in some years back. Arch finds a drug paraphernalia in his bathroom cabinet which he is disturbed by over his past as a junkie. Nikki finds a studded collar with a chain in which her reaction gives away her interest in S&M; and bondage. Nan comes out of the shower when she hears a baby cry, and she finds a tape recorder playing in her drawer which makes her upset. Outside, a drunken Skip enters the boathouse near the dock when he is grabbed by someone.The next day, Kit and Rob find Muffy in the kitchen cooking breakfast with her hair astray and acting strange. The group enjoys hanging around the back yard, while Nan tries to chase down Muffy. Rob is still upset about his own discovery in his and Kit's bedroom where his counselor informed her that he was not serious enough to get into medical school. He and Kit go to the boathouse to kiss when Kit sees Skip float by and the two of them run to tell the others. Chaz finds Skip's broken and bloody trick switchblade, but no trace of Skip's body. Arch suggests that it is Buck who has returned to seek his revenge for his being maimed the previous day.Meanwhile, Nan catches up to Muffy and angrily tells her about finding the cassette-tape joke, and Muffy claims not to know what Nan is talking about. Nan tells Muffy that she had an abortion some years back and kept it quiet between them.In the woods, as Arch is running through to the house, he gets caught in a trap when he is hung upside down by a snare, while a snake is on the ground trying to bite him. An unseen person appears, kicks away the snake, and stands over Arch who screams. On the same path a few minutes later, no one can find Arch. Rob tries to call the police on the house phone, but is unable to get through.When the water faucets in the house turn off, Hal and Nikki go to the nearby well. Hal accidentally drops the bucket into the well and does not want to go down to retrieve it. Nikki climbs down into the well when a rung breaks, making her fall into the water. As Hal climbs down to help her, Nikki finds the severed head of Arch, as well as Nan's dead body in the well.That evening back at the house, Constable Porter calls and says that he is at the hospital with Buck and tells them that he is coming to the island with an important thing to tell them and he will send a flare up at the dock when he arrives. Aware that there is a killer out there, the group goes around locking all the doors and windows of the house. In the study, Kit finds an old photograph of twins. Nikki says that Muffy has been acting strange all day and looks different from a pasty white face to her Catholic Girls School clothing as well as mentions the clodhopper "nurse shoes" that she also is wearing. Hal mentions that he heard Muffy and Nan arguing earlier about something in Nan's room, referencing an abortion that Nan was rumored to have gotten. They talk about the things found in their rooms when Muffy enters and says that she is going to bed. Chaz and Nikki go upstairs to their room as Rob heads up to the attic to keep an eye out for the constable. Kit goes off with Rob while Hal stays to watch the rooms downstairs.As Nikki packs her bags to leave, Chaz tries to calm her down by putting on a leather mask. Annoyed, Nikki pushes him to the bed and leaves the room. Nikki returns to see Chaz lying still on her bed. He does not respond to her and when she moves his hands away from his crotch, she sees blood. She is approached by someone standing behind the door.In the attic, Rob tells Kit that no one is to be left alone with Muffy for he does not trust her. They find a collection of dolls, apparently representing dead people, and a red flare goes off in the distance outside. They run out of the attic and call to everyone, but cannot find the others. The two enter Nikki and Chaz room only to find a bloody room, but no bodies. Kit runs into Muffy's room and finds Hal dead after being tied up and hung.In a panic, Kit and Rob run outside and to the dock where they pull in a motorboat, but no one is on board. They find a letter on the boat addressed to Constable Potter which talks of "Miss St. John", a dangerous mental patient who escaped several days ago from a clinic. Kit says it can't be Muffy for she was with her at Vassar all that time. They try to start up the boat to leave, but the keys are gone having been taken by Constable Potter who has gone off. Rob says that there are a spare set of keys to the boat in the house and the two of them return to find the house totally dark with all the doors and windows closed, and the shades pulled down. They enter through a cellar window where they see a bloody trail and find Muffy's clothes from the night before in the basement fireplace. Height measurements are found on one of the basement walls for 'Muffy' and 'Buffy'. Kit recalling the photo of the twins, says that it must be Muffy's twin sister, Buffy, who has been doing all the killing.Kit and Rob see eyes looking at them from behind a painting and, after pulling it away, discover Muffy's severed head. They run up to the kitchen to find the keys for the boat when Muffy/Buffy appears at the back door armed with a large breaking knife, trying to get inside. Buffy forces the door open and pushes Rob into a closet and locks it and goes after Kit, who runs into the dining room and tries to reason with the pasty-faced, wild-eyed Buffy who does not respond and terrorizes her with the breaking knife. Kit pushes Buffy back when she tries to stab her and runs into the adjacent living room to find all of the dead people, alive and well, sitting around the room. Buffy walks in and.... pushes the fake blade of the breaking knife into her hand as she everyone else looks up at Kit and yell... APRIL FOOLS!!!!!!Buck appears in the closet with Rob, pulls off his prosthetic injury, and slaps it on Rob's face and unlocks the closet door for him. Rob runs out and into the living room where everyone laughs some more. Kit and Rob are relived, but not amused at this giant practical joke on them. Muffy thanks all of them for participating and helping out. She tells everyone about her idea about making the house into a country inn, but having a special type of "whodunit weekend" where guests are to figure out a simulated mystery and she wanted a "rehearsal". Once a person was "killed" they were let in on the joke, with Rob and Kit being the last ones left and the winners who figured it all out. Muffy explains that her twin sibling is actually her cousin Skip, Constable Potter is actually her wealthy uncle, and Buck is a make-up artist who constructed all the severed heads and fake bodies. Even the ferryman was in on the whole thing as well. Muffy apologies for some of the more personal jokes, and the group drinks from dozens of bottles of champagne that Muffy brings out and they all have a happy good time.Later that night, a very inebriated Muffy returns to her bedroom for the night when she finds a package on her bed and upon opening it up finds a jack-in-the-box. As Muffy warily beings turning it to open, the jack jumps out when suddenly Nan jumps out behind the bed, grabs Muffy by the hair, gorily slices her throat, and Muffy screams. Nan smiles and shows Muffy the fake knife with the fake blood spraying from it, as a final back-at-you joke. "April Fools" says Nan to Muffy, as the jack-in-the-box winks at the viewers.
Output: Question: Whose severed heads does Nikki find?, Answer: Arch's severed head