Hannah, a high school senior and aspiring nurse, is on spring break at a resort in Puerto Vallarta, Mexico, with her best friend Emmy Cole, Emmy's parents, and Emmy's older brother Jackson, a college sophomore studying criminal science at Portland State. Hannah has harbored a crush on Jackson for a decade, complicated by a kiss they shared the previous Christmas Eve that Jackson abruptly called a mistake the next day. Emmy, who plans to skip college for a solo world tour starting in Italy, knows nothing about it.
While swimming, the girls meet Ben, a charming, wealthy boy staying at the resort who quickly shifts his attention to Emmy. Over the next two days, he charms the group and proposes chartering a private sailboat for their final full day. Emmy's parents agree on the condition that Jackson chaperone. On the morning of the trip, storm clouds line the horizon and every licensed charter tour has been canceled. Ben reveals he hired Captain Keith, a mid-30s American who lives on his sailboat and does informal private trips, by walking the docks and offering $1,000. Jackson objects, but Emmy refuses to leave, and Hannah boards last.
As they sail through Banderas Bay, Captain Keith explains that the Sierra Madre mountains make the coastline between towns virtually impassable by land, meaning anyone stranded on a remote stretch of shore is effectively marooned. During conversation, Ben insults Hannah's nursing aspirations and invites himself on Emmy's world tour. Emmy eagerly agrees, stinging Hannah, who warns her about traveling with a stranger. Ben and Emmy dismiss Hannah as overly cautious.
The afternoon turns dangerous when a lightning strike reveals the storm is directly overhead. The group discovers that Captain Keith has abandoned the helm, or steering position, and passed out drunk in the bathroom. Ben drags him on deck and beats him, breaking his nose. When Keith threatens to press assault charges, Ben acts. Standing at the wheel during a strong gust, he deliberately releases it. The wheel spins uncontrolled, and the boom, the horizontal pole holding the bottom of the sail, swings violently, strikes Keith in the head, and flings him into the ocean. Hannah dives in but finds no trace of him. Back on the boat, Ben insists it was an accident and demands the group agree on a unified story. Hannah tells him she watched him let go on purpose.
Below deck, their phones have no service and the boat's battery is dead, leaving the radio and navigation systems powerless. During the night, Jackson catches Ben cutting through every radio and navigation wire with a folding knife, destroying their only potential means of communication. Ben admits he did it to prevent them from contacting authorities before agreeing to cover for him. Hannah and Jackson lock Ben in the bathroom. Emmy reluctantly agrees after reckoning with Ben's escalating violence.
By morning, the mast has snapped, the motor will not start, and they are fully adrift. Emmy privately reveals that her continued warmth toward Ben is an act designed to extract useful information. During a tense exchange, Jackson finally explains that his father warned him a failed relationship with Hannah could cost her the Coles as her second family, which is why he retreated after the kiss. He confesses he regrets his cowardice and has thought about Hannah constantly. They kiss, and Emmy walks in, furious about the secret.
Ben escapes the bathroom by setting toilet paper on fire with a concealed lighter, then puts Hannah in a chokehold. She breaks free, and Jackson punches Ben off the boat, refusing to lower the ladder until a terrified Ben agrees to cooperate. They tie him to the broken mast. The second storm arrives with catastrophic force. They cut Ben free before lightning can strike the metal, and he immediately tries to steal Hannah's life vest. Emmy smashes his head into the cabin wall. Jackson and Hannah shove Emmy into the cabin, but before they can follow, an enormous wave crashes into the boat.
Hannah regains consciousness floating in the water, tied to the capsized vessel by a rope she does not remember securing. She has a torn flap of scalp, extensive bruises, and cuts. She hears rhythmic knocking inside the hull and finds Emmy in a small air pocket with a severe gash from wrist to elbow, already infected, still clutching their bag of food. Hannah dives through the submerged hatch and drags Emmy out. She then spots Jackson in the water, clinging to the emergency flotation ring. Emmy has a seizure. Hannah retrieves a first aid kit and an emergency life raft from the sinking boat, inflates the raft, and they spot land on the horizon.
Hannah and Jackson kick the raft toward shore for hours, Hannah swimming through a swarm of jellyfish that leaves welts across her body. They reach a remote, uninhabited beach backed by dense jungle. Hannah finds a freshwater stream and fruit, treats Emmy's wound with improvised bandages, and repeatedly lowers her fever in the cold water. Her attempts to build a signal fire fail with damp wood. Her bag of food vanishes overnight. She hallucinates a helicopter that Jackson confirms was not real. She eventually builds a massive bonfire with drier wood from the jungle interior and carves "HELP" in the sand.
That night, a shirtless figure emerges from the jungle, stabs the raft, kicks sand at the fire, and flees. Hannah chases the intruder the next day and discovers a makeshift shelter containing her stolen supplies and a piece of fabric she recognizes as Ben's shirt. Ben attacks her with a branch. He survived the storm on the flotation ring and explains his strategy: With all witnesses presumed dead, he could have told any story he wanted. Hannah's competence in keeping Emmy alive threatened his plan, so he stole their food, stripped the fruit trees, destroyed the raft, and smothered the fire to ensure they died before rescue came.
During their violent struggle up a rock outcropping, Ben reveals that Jackson is not on the beach. During the second storm, Jackson used Ben's knife to cut a rope and tie Hannah to the boat for safety. While Jackson secured the knot, Ben took the knife and stabbed him in the neck. Hannah's mind reels as she replays every interaction with Jackson since waking on the capsized boat. He never touched anything physical, never ate or drank, never carried Emmy, never left footprints. Her exhausted, traumatized brain fabricated his presence to keep her going.
Ben tries to drag Hannah off the cliff onto jagged rocks below. She challenges his logic: His story collapses if three people die of exposure while he survives unscathed. When he dismisses her, she trips him. He falls and dies on impact. A real helicopter appears, drawn by her smoke signal, and rescue arrives.
In an epilogue set almost a year later, Hannah is a freshman at Linfield University's nursing program, grieving Jackson and avoiding home because everything there reminds her of him. Emmy recovered from her injuries, nearly losing her arm to infection, and departed on her world tour. Ben's body was never found, nor were Captain Keith's or Jackson's. Emmy surprises Hannah at college and invites her to spend spring break in Italy, reclaiming the destination Ben had tainted and honoring Jackson. Hannah agrees, with one condition: no boats.