Maddie, a talented cellist, is trying to move forward after a tragedy in which her friend Chad died protecting her during a robbery, and the incident left her with hearing loss she manages with hearing aids. To help her heal, her best friend Trish, a PhD student studying gene editing and a social media influencer, surprises her with a luxurious vacation to Thailand.
After flashing her waterproof phone, Trish encourages Maddie to visit a local attraction featuring Ceto, a captive orca. Maddie, who deeply disapproves of the captivity, refuses to fund the venue. Instead, the trio—accompanied by an expat named Josh—drinks, jokes, and ultimately breaks into CetoWorld’s water park. Maddie is thrilled to see Ceto through a viewing window, but the moment quickly turns grim when a janitor falls into the tank and Ceto kills him. In the aftermath, park management decides to release Ceto back to the ocean.
The following day, the group explores a remote atoll lagoon marked by a looming sea stack, a place locals claim is haunted. Their excursion spirals into a nightmare when Josh and Trish are ambushed by Ceto while riding a jet ski; Ceto kills Josh, and Trish clings to Maddie’s inflatable. The two women paddle toward an islet but must swim the rest of the way. Climbing the rock, Maddie cuts her foot, and Trish slashes her leg. They realize Ceto is attacking them because the orca has become confused after accidentally entering the lagoon’s rock perimeter, mistaking them for her former trainers at CetoWorld.
To survive the next days, the pair survive on gum and an oyster, and drink water from a sun-warmed plastic bag. They call out for a nearby boat, grasping Josh’s arm to get his mini flashlight and signaling at night. With dwindling hope, Maddie drops Chad’s cremation stone into the water, hoping it might ease her pain. Guilt overwhelms Trish, who finally admits that she orchestrated the robbery to pay off a placement debt—though she didn’t intend for the gun to go off, for Maddie to lose her hearing, or for Chad to die.
Anger flares, but Maddie is forced to reassess. The next day, Trish realizes that if she can swim to shore, she can distract Ceto long enough for Maddie to reach another islet near the lagoon’s edge. Their plan works momentarily as a plane passes overhead and Trish signs SOS in the sand. Yet the distraction costs Trish dearly when Ceto bites off her leg, and she bleeds out.
At night, Maddie’s hearing aids run out of power, leaving her unable to hear. She sees and speaks to Ceto, then chooses to swim toward the perimeter once more. Ceto refrains from attacking, and Maddie finally reaches the sand bank, where she buries Trish. She searches for Trish’s bag containing the waterproof phone; the phone dies again, and Ceto closes in. Maddie retrieves her cello endpin and, in a final, desperate measure, stabs Ceto in the eye. The injured orca retreats, whimpering. After drifting for a time, Maddie is rescued by a helicopter. Months later, she returns to the stage and performs a cello piece, though one seat remains empty in the audience, a quiet reminder of the cost of survival.
