Plot
Maddie, a talented cellist, is struggling to recover from a tragedy in which her friend Chad died protecting her during a robbery. A gun misfire during the robbery also left Maddie with hearing loss for which she uses hearing aids. To help her heal, her best friend Trish, a PhD student studying gene editing and social media influencer, surprises her with a luxury vacation to Thailand.
After showing off her waterproof phone, Trish encourages Maddie to visit a local attraction featuring Ceto, a captive orca. Disapproving of the captivity, Maddie refuses to support the venue financially. Instead, the pair, accompanied by an expat named Josh, get drunk later in the night and break into Ceto's water park. Maddie is pleased to meet her through a viewing window, but is horrified when a janitor falls into the tank and Ceto kills him. The trio flees, while the park management decides to get rid of Ceto by releasing her into the ocean.
The next morning, the trio decide to explore a secluded atoll lagoon demarcated by a sea stack, which locals say is haunted. Their excursion turns into a nightmare when Josh and Trish, who were riding a jet ski, are ambushed by Ceto, who kills Josh while Trish manages to reach Maddie's inflatable. The women start paddling to reach an islet, but are forced to swim the rest of the way. Climbing the rock, Maddie pierces her foot and Trish slashes her leg. The pair deduce that Ceto is attacking them because she has become confused after accidentally swimming into the lagoon's perimeter of rocks, and (thinking she is still held captive) cannot distinguish them from her former CetoWorld trainers.
In order to survive the next days, the pair eat gum and an oyster, and drink water from a sun-stroked plastic bag. They also call out for a nearby boat and reach out to Josh's arm to get his mini flashlight and make signals at night.
Thinking she might not survive, Maddie drops Chad's cremation stone into the water. Overcome by guilt, Trish confesses to Maddie that it was she who planned the robbery to pay her placement debt, even though she didn't mean for the robber to be armed, for Maddie to lose her hearing, or for Chad to die. Enraged, Maddie ignores Trish until the next day, when Trish realizes that if she can swim to shore, she can distract Ceto enough for Maddie to reach another islet near the perimeter. Their plan is successful, and while a plane flies over them, Trish makes an SOS sign on the sand. Unfortunately, while she is distracted, Ceto bites off her leg, and she bleeds out.
At night, Maddie's hearing aids run out of battery, leaving her unable to hear. Maddie sees and speaks to Ceto and then makes the decision to swim to the perimeter. Ceto does not attack Maddie this time, and Maddie successfully reaches the sand bank. There, she buries Trish. Then, she sets off to look for Trish's bag with the waterproof phone. She finds the phone, but it shuts off and Ceto approaches again. Maddie retrieves her cello endpin and stabs Ceto in the eye. Ceto swims away, whimpering. After floating in the water for some time, Maddie is rescued by a helicopter. Months later, Maddie performs a cello piece and there is one empty seat in the audience.
