Happy Birthday to Me

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Virginia "Ginny" Wainwright is a pretty and popular high school senior at Crawford Academy. She is one of her school's "Top Ten," an elite clique which comprises as the richest, most popular and most snobbish teens at the Academy. The Top Ten meets every night at the Silent Woman Tavern, a pub near Crawford's campus.
One night en route to the tavern, Top Ten member Bernadette O'Hara is attacked in her car by a killer whose face cannot be seen. Unable to flee, she struggles then plays dead to catch the killer off-guard before running to get help. She then finds somebody whom she is familiar with only to have her throat slit by the person with a straight razor (whom the audience still cannot see) who is actually the killer.
The Top Ten becomes concerned when Bernadette fails to show up at the Silent Woman. They soon get over it when their argument with another customer results in one of them putting a pet mouse into his beer. Mayhem ensues and the Top Ten flees the scene.
On the way home, the Top Ten see a drawbridge going up and decides to play a game of chicken. All cars in the game tempts to make it across before the bridge is completely raised (to allow the passing of ferries). A protesting Ginny is shoved into a car by fellow Top Ten member Ann Thomerson. Every car jumps the drawbridge save one. As the car goes over the drawbridge, Ginny yells "Mother!" and makes it across safe. After the car stops, Ginny runs from the vehicle into the darkness home.
On the way home she stops by her mother's grave to tell her she's popular and hangs out with the Top Ten all the time. Ginny is confronted by her father about coming home after her curfew. Unbeknownst to either of them, Top Ten member Etienne Vercoures, a French foreign exchange student has followed her home before she was encountered by him. He enters Ginny's room and steals a pair of her underwear and escapes without being seen. She then discovers her window was open. As she runs to slam it shut, she heard a noise and screams. The next day when she and Ann arrived late at school to class, school principal Mrs. Patterson is disturbed by the behavior of the Top Ten members and confronts them.
Ginny shares a handful of lost repressed memories with her on-call psychiatrist, Dr. David Faraday. She underwent an experimental medical procedure, involving surgery to restore brain tissue, after surviving a harrowing accident at the drawbridge.
As Ginny attempts to resume her normal life, her fellow Top Ten members began been murdered in vicious and violent ways: Etienne got strangled when his scarf gets caught in the spokes of his motorcycle as blood splatters. (Ã  la Isadora Duncan) and Greg has his neck crushed while lifting weights. And yet, the killer who always sports a pair of black gloves is never seen. Until one night, Top-Ten member Alfred, who's had a crush on Ginny, follows her to her mother's grave and is then stabbed with a pair of garden shears by Ginny.
On the weekend of her 18th birthday, Mr. Wainwright leaves to go on a business trip. After a school dance, Ginny invites Steve, another member of the Top Ten to her place for a midnight snack. She prepares shish-kebabs and feeds him while they drank wine and ate. Steve then unsuspectingly leaves his mouth open when Ginny shoves the kebab skewer violently down his throat leaving little doubt that Ginny may be the killer at this point.
The following morning, when Ann arrives at her house, Ginny is taking a shower fighting to remember everything up to that point. In flashbacks, the following is revealed that Ginny's mother, a newly inducted socialite, invited the Top Ten to Ginny's birthday celebration four years earlier. Instead, the Top Ten went to Ann's party. Drunk and unstable as Mrs. Wainwright confronted the groundskeeper. Ginny's mother learned that she had a reputation as the town whore. Neither, she nor her daughter were welcome at the Thomersons. This led to Mrs. Wainwright attempts to drive across a bridge that was in the process of opening. In the middle of a rainstorm, with Ginny screaming, her mother finally stopped in the middle of the bridge as each side are still raising. The car fell in between the bridge halves as Mrs. Wainwright drowns in the car, though Ginny swam to safety.
Ginny realizes that she may have killed her friends after all, including Ann who had just paid her a visit. With Ginny's 18th birthday steadily approaching, she struggles to get answers from Dr. Faraday. When he fails to provide any, she kills him with a fireplace poker.
Mr. Wainwright returns home from his business trip ready to celebrate his daughter's 18th birthday. When he entered the house, he sees blood and frantically flees the house and attempts to locate Ginny. Instead, Mr. Wainwright finds Greg's girlfriend Amelia in the middle of a rainstorm in shock in the courtyard clutching a gift. His late wife's grave (which recently has been robbed) with Dr. Faraday's corpse in it. Entering a cottage which serves as the Wainwrights' guest quarters, he makes a ghastly discovery.
The corpses of all the murdered Top Ten members are seated around the table which has been set to look exactly as it did four years ago. The corpse of Mrs. Wainwright is also seated there as well. Ginny then enters carrying a large cake and singing "Happy Birthday" to herself. Already distraught, Mr. Wainwright bursts into tears when his daughter casually admits to committing the murders. Ginny slits her father's throat with the same large knife as she was going to use to cut the cake. He never sees the real Ginny who is sedated, seated at the table as the killer's only living guest.
Ginny's doppelganger rants about having done all of this for Ginny, who then awakens to discover that the second Ginny is really Ann in disguise. Ann has been embittered by the revelation of her father's affair with Ginny's mother. It turns out that both girls are half-sisters. Ann slaughtered the six main members of the clique who never showed up for her birthday party. Expressly for the purpose of framing Ginny who suddenly breaks free, takes Ann's knife and kills her with it.
As she stands over her half-sister's corpse, a detective walks in on Ginny and the horrifying carnage that surrounds her. He stares at Ginny and demands: "What have you done?" The audience is then left to wonder if Ginny will be cleared of the killings or arrested for them. The film ends as Ginny singing "Happy Birthday To Me" to herself as the screen cuts to black.[citation needed]

Question: Who yells "Mother!"?
Answer: Ginny