Prom Night III: The Last Kiss is a 1990 direct-to-video horror-comedy film that is part of the "Prom Night" film series, continuing the storyline involving the murderous ghoul Mary Lou Maloney. This is the only entry to depart from the strictly horror genre of the series, and is instead a spoof of the previous films. This film was followed by a third sequel, titled Prom Night IV: Deliver Us from Evil.
Trapped in Hell, murderous prom queen Mary Lou Maloney (Courtney Taylor), who burned to death in 1957, manages to escape her chains by severing them with a nail file. Returning to her place of death, Hamilton High School, Mary Lou kills the school janitor and one of her many former boyfriends Jack Russell (Terry Doyle) by electrocuting him with a jukebox to the point that his pacemaker bursts from his chest. The day after Jack's death, Principal Weatherall (Roger Dunn), officially opens Hamilton High's recently reconstructed gymnasium, accidentally severing one of his own fingers while cutting the ribbon with a pair of scissors, an act which prompts an unseen force to wreak havoc through the gym with powerful winds.
Hours after the gymnasium opening largely average student Alexander Grey (Tim Conlon), who dreams of going to medical school, leaves a date with his girlfriend Sarah Monroe (Cynthia Preston) to get his textbooks from school to study for an upcoming test, having been told by snarky guidance counselor Ms. Richards (Lesley Kelly) that his grades mean he will never reach medical school and be left to do little more than menial labor. While in the school, Alex is approached by Mary Lou, and the two ultimately have sex on the American flag in a hallway. Waking up, Alex redresses and, throughout the day, Mary Lou appears to him, both during his biology test and during a football game, which Mary Lou helps Alex win, much to the anger of Alex's rival Andrew Douglas (Dylan Neal).
When biology teacher Mr. Walker (George Chuvalo) fails Alex's test, Mary Lou murders him, pinning his hands to a table with ice cream cones before stabbing him in the face with a mixer and stuffing him with food, mostly fruit. Finding Mr. Walker's body, Alex hides it in a cupboard and at night buries it in the football field before going off to have sex with Mary Lou, who seduces him in the guise of a scantily clad nurse.
With Mary Lou's help, Alex's grades skyrocket and he makes the honor roll and becomes a football star, though his secret romance with Mary Lou also strains his relationship with Sarah. After Mary Lou burns Ms. Richards to death with battery acid after the counselor becomes suspicious of Alex's grades, Alex, having received a motorcycle and leather jacket from his parents as gifts for his achievements in school, buries Ms. Richards’s body in the football field. After disposing of Ms. Richards, Alex is confronted by Andrew, who had earlier kicked him off the football team, and the two get into a fight, which ends when Mary Lou kills Andrew by impaling him to the football goal post by hurling a football, which changes into a spinning drill in mid-flight, at him. Growing tired of Mary Lou's murders and her obsession over him, Alex tries to break things off with Mary Lou, which enrages the ghost.
Trying to go on with his life after dumping Mary Lou, Alex tries to patch things up with Sarah by asking her to the prom inaugurating the new gym, only to learn she is going with nerdy Leonard Welsh (Jeremy Ratchford). Finding himself stalked by Mary Lou, Alex tells his best friend Shane Taylor (David Stratton) everything, which prompts Mary Lou to kill Shane by ripping his heart out. Shane's death is then blamed on Alex, who Shane's parents see fleeing from their house with blood on his hands. Tracked down to his house, Alex is arrested and put in jail. While in his cell, Alex is approached by Mary Lou who, after Alex rejects her once more, leaves to kill Sarah, electrocuting a pair of officers and leaving behind the keys to Alex's cell, which Alex uses to escape.
As Alex races to the prom, forcing Officer Larry (Brock Simpson) to drive him there at gunpoint, Sarah is attacked by Mary Lou, who had killed Leonard by wrapping him in magnetic tape. Reaching the gymnasium as Mary Lou is about to kill Sarah on stage, Alex willingly goes to Hell with Mary Lou, making her promise that if he goes with her she will leave everyone else alone. As Mary Lou and Alex descend into the ground, Sarah follows them, jumping into the portal before it closes. Fighting off zombified versions of Shane, Leonard and Andrew in a nightmarish version of Hamilton High, Sarah tracks Alex down to Hell's equivalent of Hamilton High's gym, where Mary Lou is about to kill Alex so he can be her prom king for all eternity.
Managing to briefly incapacitate Mary Lou by blowing her up with an unstable blow torch, Sarah and Alex avoid the swarms of zombie students after them and reach a car in the Underworld school's garage, which Sarah hotwires. Before the zombies overwhelm them, Sarah and Alex seemingly escape back to Earth by hitting Mary Lou with the car. Driving to a payphone at a diner, Alex, realizing the phone only takes dimes, asks Sarah for some, and witnesses her be killed by Mary Lou, who drives her hand through Sarah's back and out her chest. With Sarah dead, Alex notices he is trapped in what appears to be the 1950s and, realizing no one can see or hear him, loses his sanity and goes into hysterics, screaming "You won, you fucking bitch!"
The DVD version released in a double feature DVD along with Prom Night IV: Deliver Us from Evil is a cut TV broadcast version. Most of the gore, swearing, and nudity is all but removed. However in the VHS version it is uncut, with all gore, profanity and nude scenes left intact. The uncut version of this sequel has been released on DVD in other countries such as the United Kingdom, Germany and Australia.