Plot
For the past few years, a serial killer known as "Heart Eyes" has moved from different cities in the United States, killing multiple couples on Valentine's Day. In the present year, Heart Eyes moves to Seattle and murders two couples; one at a winery and another at a spa.
Elsewhere, Ally, a pitch designer for a jewelry company who has recently broken up with her boyfriend, is being ridiculed for a commercial she wrote involving doomed couples, seen as offensive due to the recent string of murders. Ally's boss, Crystal, orders her to work with famed designer Jay Simmons, whom Ally had encountered at a coffee shop earlier. Jay suggests they have dinner that night to gain inspiration, and Ally's best friend Monica helps her pick out an outfit.
Ally and Jay meet at the restaurant, but the conversation gets heated when they question each other's status and backstories. Jay leaves, and Ally follows, kissing him as she sees her previous boyfriend approaching with his new girlfriend. From a distance, Heart Eyes watches. The two of them reconcile their differences and take a cab to Ally's apartment, where Jay helps her break in after she forgets her key, cutting his hand in the process. Ally helps tend to Jay's wound upstairs, only for Heart Eyes to attack them. He kills their cab driver and chases them, knocking Jay out in the process. Ally flees and ends up on a carousel before the police arrive. Detective Jeanette Shaw and her partner Hobbs arrest Jay for the murders due to evidence regarding a wedding ring engraved with the initials "J.S." at the winery where Heart Eyes murdered the first couple, though Jay claims his innocence.
At the station, Shaw and Hobbs point out how Jay's initials match the one on the ring, and Shaw points out that he was in the same cities at the same time as the killer's previous attacks. Ally waits in the lobby with the desk clerk and meets David, who was working on the computers. David asks Ally out on a whim, but Ally declines due to her recent stress, and David leaves. Heart Eyes invades the station by deactivating the power and killing the clerk before moving in to attack Ally. Shaw confronts Heart Eyes who disappears while Hobbs is killed. Ally rescues Jay, and they flee to a drive-in movie theater, where Heart Eyes slaughters several movie-goers. They decide to confront the killer. In the scuffle, Jay manages to stab Heart Eyes, and they unmask him to reveal an unknown man. The two of them are tended to by an ambulance while Shaw arrives with a hurt arm. She offers Jay a ride home, and he accepts.
At home, Ally realizes she loves Jay and goes to find him at the airport. However, she is unable to find him until Heart Eyes calls her, telling Ally that he’s holding Jay captive at a chapel. Ally arrives, and Heart Eyes unmasks himself as David, who reveals that the previous Heart Eyes was a copycat and fanboy named Eli. Shaw then reveals herself as another Heart Eyes copycat and David's wife. Both she and David have committed every murder as Heart Eyes, switching out the costume. Shaw, in particular, attacked them at the apartment, while David killed the cab driver and the couple at the winery, losing his ring. Their motivation is a kink for killing together for fun. They present Ally with the choice of killing herself or Jay, but she shoots Shaw instead, hitting Jay in the process.
The two couples fight as Jay stabs David in the eye with his own arrow, resulting in David falling on candles where the wax burns him. Ally then pushes Shaw onto a statue of Saint Valentine, stabbing her in the neck with a metal straw, as it slowly decapitates her. At the same time, Jay shoots David with another arrow, finally ending his and Shaw's killing spree. One year later, Ally returns to med school and the couple attend a drive-in movie. Jay asks Ally to move in with him, but she instead proposes and he accepts.
In a mid-credit scene, Ally receives a mysterious and sinister phone call, which is then revealed to be Monica pretending to be Heart Eyes, having photographed the proposal.
