After escaping from "Bad Girl Hell", where she had been imprisoned for her crimes in Hello Mary Lou: Prom Night II, dead debutante Mary Lou Maloney (Courtney Taylor) returns to Hamilton High to continue her reign of terror as the 'Prom Queen From Beyond The Grave'. She wants revenge and she's looking for a victim... any victim.
Enter Alex Grey (Tim Conlon), an average high school student whose mediocre grades and lacklustre performance on the football field rank him somewhere near the bottom of the school pecking order. With his future plans of medical school and the prospect of marriage to girlfriend Sarah Munroe (Cyndy Preston) in jeopardy, Alex is looking for something new in his life. As if in answer to his prayers, along comes Mary Lou. But murder isn't on her mind when she meets Alex. To her surprise, she falls head over spike heels in love.
At first, it seems too good to be true. Alex has met the girl of his dreams and somehow she manages to increase his grade average and helps him score both on the football field and under the sheets! But while researching the history of the school, Alex discovers the truth about his secret lover... she has been dead for over 30 years! Still, nobody's perfect, and Alex is willing to continue his ghostly affair. That is, until he discovers the dead bodies of his science teacher (George Chuvalo) and guidance counsellor (Lesley Kelly), victims of bizarre '50s-style murder scenarios - Mary Lou's way of dealing with Alex's sagging grades. And as he soon discovers, it's up to Alex to hide the bodies.