Tree Gelbman (Jessica Rothe) awakens in the dorm room of her classmate, Carter Davis, disoriented and oblivious to the previous night’s events. She ignores a call from her father and forbids Carter from mentioning her presence to anyone. Ryan Phan (Phi Vu), Carter’s roommate, bursts in and sees her just as she is about to leave.

On her way back to the sorority house, Tree dismisses a woman seeking her signature for a global warming campaign. She encounters Tim, who has been waiting for her in a dark corner to inquire about her failure to respond to his text messages. She insults him, recalling his decision to take her to Subway on their first date. Her roommate, Lori Spengler (Ruby Modine), presents her with a homemade cupcake for her birthday, which Tree discards in the trash, claiming its excessive carb content.

Tree visits Bayfield University Hospital and accidentally runs into Lori, who seems to be aware of her purpose for being there. Lori warns Theresa that her behavior is wrong and will have consequences, but Theresa dismisses her advice, claiming it is none of her concern. Lori’s secret affair with Dr. Gregory Butler is interrupted by the unexpected arrival of his wife.

That night, as Tree walks through the dark tunnel towards the party, she is drawn to a rotating music box on the floor. Intrigued, she approaches the object, only to be ambushed and later stabbed to death by a masked figure wearing the school mascot mask. The world fades to black.

To her surprise, she awakens in Carter’s dorm room, hungover. Assuming it was just a dream, she dismisses the events that transpired, yet everything unfolds precisely as they did before. But tonight, she decides to avoid the tunnel and safely arrives at the party. However she is murdered again by the killer who follows her to the party.

Again, Tree awakens in Carter’s bed, realizing she is trapped in a temporal loop. Despite her attempts to conceal herself or flee, she is inevitably murdered or succumbs to an accident. Even suicide fails to provide an escape, as she always awakens on the same day. Tree explains her predicament to Carter, who suggests exploiting the loop to identify her killer, positing that it may be the only way to break the cycle.

Happy Death Day is essentially a slasher film version of Groundhog Day, incorporating contemporary elements to maintain audience engagement as they root for the protagonist to escape the time loop and devise methods to identify the true killer. The film is a thrilling ride from beginning to end, although it leaves the circumstances of the protagonist’s initial entry into the time loop unexplained, perhaps for a future sequel.

HAPPY DEATH DAY was theatrically released in the United States on 13 October 2017. The film grossed over $125 million worldwide against a $4.8 million budget.