Jess plans a boat trip for her and her autistic son, Tommy, inviting her friend Greg along. She tries to reassure Tommy, telling him he just had a bad dream, before taking him to school and heading to a Florida harbor without him. On Greg’s boat, they’re joined by Victor, Sally, Downey, and Sally’s friend Heather. A quiet pre-dawn moment on the water becomes unsettling as Jess has a dream of crabs on a beach and awakens disturbed, though she can’t quite pin down what unsettled her.
A sudden storm moves in, and Greg radios the Coast Guard while a distress signal drifts from a woman claiming someone is killing everyone. The storm escalates and the sailing vessel capsizes. Heather is swept away, and the survivors cling to the overturned sailboat as the weather finally clears.
They spot a passing ocean liner, the Aeolus, and the group boards it, finding the ship oddly deserted despite a shadowy figure they thought they’d seen aboard. A creeping sense of déjà vu settles over them as they move through the quiet corridors. Keys and fresh food appear in the dining room, suggesting someone has been there, yet no one greets them. One by one, they notice someone watching them, and Victor gives chase when Jess returns to the dining room to discover something chilling: the words “go to theater” written in blood on a mirror. Greg dismisses it as a prank, but the unease continues.
Tension spikes when Victor, injured, makes a bid to strangle Jess, and she fights back, forcing him to collapse. Gunfire soon erupts, and in the theater of the ship, Greg lies dead from a gunshot. Sally and Downey claim Jess killed him, while a masked assailant in a burlap mask shoots Sally and Downey. Jess seizes an ax in a desperate counterattack, disarms the shooter, and is confronted with a chilling line: > You have to kill them; it's the only way to get home.
As the mystery deepens, Jess encounters a terrifying twist: another version of their group, identical in appearance, boards the Aeolus. This second cohort spots Jess as she drops her keys, and she tries to warn a newly arrived Victor, only to knock his head against a wall hook in the process. She discovers dozens of lockets bearing handwriting that matches her own, urging her to kill everyone. Desperate to halt the loop, she arms a shotgun and confronts the new group with the intention of “changing the pattern,” but a third Jess shoots the new Greg and stabs the new Downey and Sally, fracturing the sequence further.
The original Jess pursues the second Sally, who sends out the distress signal heard on Greg’s boat. On a deck crowded with corpses and shadows, the two Jess-es collide in a deadly chase as the third Jess is killed by the one who started it all. The overturned vessel reappears with another set of duplicates, and the old Jess realizes the loop resets whenever all copies are slain.
Determined to break the cycle, she sinks her own memory into the ship’s mirror with blood, discards bodies overboard, and directs the latest Sally and Downey toward the theater. She then dons the shooter’s garb and an eerie burlap mask, preparing for a final confrontation. After being briefly disarmed, she urges the newest version of herself to kill everyone, before she is knocked overboard.
She awakens on a beach, near crabs, having washed ashore at an earlier moment. A return home follows, and she witnesses her earlier self erupting in anger and striking Tommy. She distracts the aggressor with a doorbell, then kills her other self and tells Tommy that he merely had a bad dream. With a bagged body in the trunk, she leaves with Tommy, promising to change their lives. A seagull’s death on the windshield hints at the endless loop she’s trapped in, and a pile of dead seagulls soon appears as she drives away, exhausted. Yet the road drama ends in tragedy when a collision with a truck kills Tommy. A taxi driver offers to take her to the harbor, and she vows to return, but the cycle resumes as she, emotionally spent, re-joins Greg’s boat and the loop starts anew.