The story unfolds in a girls' school located in Yucatan, Mexico, where the drama begins when Mia (Sophie Nélisse) finds herself pushed into a swimming pool by the cruel Catherine (Brec Bassinger). To make matters worse, Mia's stepsister Sasha (Corinne Foxx) stands by and denies their sisterhood. Their mother, Jennifer (Nia Long), who is married to Mia's dad Grant (John Corbett), comes to pick them up. During their evening meal, Grant shares an exciting find—a shark tooth from one of his cave-diving expeditions—which he gifts to Mia, as her fascination with discoveries aligns with his own.
The tension escalates when the girls learn that they are scheduled for a boat trip to observe sharks, much to Sasha's dismay, who had hoped to spend time with her friends instead. Grant, accompanied by his assistants Ben (Davi Santos) and Carl (Khylin Rhambo), drops off the girls while embarking on another diving venture. However, Sasha's friends Alexa (Brianne Tju) and Nicole (Sistine Rose Stallone) arrive to rescue Sasha from the oppressive atmosphere, persuading Mia to join them in fleeing from Catherine.
They find themselves by a cave along a cliffside, exuberantly diving into the water. As they explore, they don scuba gear and venture into the ancient Mayan ruins. Their discovery of old carvings and skeletons takes a dark turn when they encounter a blind Mexican Tetra, which unnerves Nicole. A commotion ensues, causing the group to split. In no time, they face a terrifying shark that attacks Ben, leaving them in a state of panic.
As the relentless shark continues to pursue them using its keen sense of hearing, the girls find fleeting moments of safety yet uncertainty lingers. They attempt to locate Grant and Carl using a guideline but soon realize they've strayed too far. After finding temporary refuge in an air pocket, they gather their wits, knowing their oxygen is running low.
Mia, following a potential escape route, inadvertently leads to tragedy when Carl meets his grim fate to the shark. The tension heightens when she finally reunites with Grant, and together they confront the looming menace. Using a bell's sound to distract the predator, they manage to escape to the cave's surface.
Facing a dire situation, they must ascend a harness, but panic ensues. Nicole slips as sharks encroach, resulting in a life-or-death struggle. Tragically, she is taken before their eyes. Grant puts himself on the line, but fate turns cruel as he is caught by the shark, leaving Mia and Sasha shattered.
With their group dwindling, Mia and Sasha, alongside Alexa, desperately try to navigate their way to safety, only to face more horrors. After a harrowing series of events, Mia and Sasha find a boat, but not before more chaos unfolds. Amidst the chaos, Mia discovers her resourcefulness and bravery, wielding a flare gun to fend off danger.
In this dark and thrilling adventure, Mia and Sasha emerge as the last survivors, receiving medical attention after a catastrophe that tested their resolve and bonded them like never before. As they heal from their traumatic ordeal, their gratitude for one another shines through, proving their sisterhood is stronger than the challenges they faced.
