Prom Night

This film is commonly cited as a crucial inspiration for 1997’s I KNOW WHAT YOU DID LAST SUMMER and its sequels.

Short pooper:
Alex (Michael Tough) is the killer; he was also the one who witnessed the death of his twin sister Robin (Tammy Bourne) as a child. He is killed by the Police and dies in the arms of his older sister Kim (Jamie Lee Curtis), crying out loud that the others killed Robin.
Longer version:
The night of their prom, Wendy (Anne-Marie Martin), Jude (Joy Thompson), Kelly (Mary Beth Rubens) and Nick (Casey Stevens) -all of whom share the painful secret, tightly kept between themselves, of having been indirectly responsible of the death, 6 years prior, of 10-year-old Robin Hammond by scaring her so badly in the middle of a twisted game of hide-and-seek in an abandoned building that she ended up falling off a second-floor window and, shortly after, the window itself detached and smashed against her- discover their yearbook pictures stabbed with a glass shard, which leads them to figure out someone knows about the accident and could be after them.

An unknown killer, wearing a ski mask that only shows its eyes, targets and kills most of the friends in various gruesome ways, all the while muttering words like “Die”, “Blood” and “Kill” every time -something the friends kept repeating, the day of the game that led to the tragedy-:
– Kelly has her throat slit with a shard of glass inside the girls’ locker room.
– Jude also has her throat slit, the same way, while Slick (Sheldon Rybowski), in the commotion, dies when the van he drove -where Jude and him were having sex- drives off a cliff and crashes.
– Wendy is cornered into a utility closet where the killer hacks her to death with an ax.

Lou (David Mucci), whom the killer mistook for Nick -the latter having been tied and stashed away, as part of a prank preventing him to claim the Prom King title-, decapitates him with the ax and his head winds up on the stage, causing a collective panic. Meanwhile, Nick’s father, Lt. McBride (George Touliatos), who had been investigating since the discovery of the “stabbed yearbook pictures” and was all but convinced that an escaped mental patient known as “Leonard Murch” was responsible of it all -only for Murch to be eventually captured and it to become obvious that the man never had anything to do with the case-, mounts guard at the prom and comes across the killing spree, alerting the Police.

Kim, who was to be crowned Prom Queen along with Nick, finds him tied up and releases him, but they cross paths with the killer and briefly struggle. Kim eventually gains the upper hand and strikes the killer in the head with the ax. The killer is stunned and momentarily locks eyes with Kim, who immediately figures out who he is and, when he runs off, she desperately chases after him.

As the killer stumbles outside grabbing his wounded head, Police officers spot him and order him to surrender, which he does not do; they get ready to shoot, while Kim, coming after them, desperately screams and begs them not to do it. She is not able to stop it and the killer gets gunned down. As he collapses on the ground, an anguished Kim removes the ski mask to reveal him as her younger brother Alex, Robin’s twin; at the same time, a flashback shows that he was the one who witnessed in shock the death of his twin sister, all those years ago.

A moribund Alex, comforted by Kim, starts crying hysterically as he calls Robin’s name and blames Jude, Wendy, Kelly and Nick for the incident, repeating: “THEY KILLED HER!!”, after which he dies. The movie ends with Kim weeping for the death of her two siblings.