Ann Thomerson is the antagonist of the 1981 mystery horror film "Happy Birthday to Me". She is one of the few female slashers in horror film history. Though not revealed until the final act of the film, Ann Thompson hates protagonist Virginia Wainwright, despite pretending for years to be her best friend. The source of her animosity stems from the fact that Virginia's mother was a mistress to Ann's father, resulting in the heroine being a bastard half-sister to the villainess, whose family was then broken up because of it. In the "present day", several of Virginia's friends and classmates are murdered one by one, in a grisly fashion. Due to past trauma that she suffered - her mother (and nearly herself) dying in an accident on her birthday, she is left in a confused and troubled state; a state that Ann takes full advantage of to make herself and those around her suspect that she is the killer, even going as far as wearing a mask of Virginia's face as she commits the murders.