Misty Calhoun, a shark conservationist, is invited to visit the sea-based facility Akhelios by pharmaceutical billionaire Carl Durant. She travels with neurobiology students, Leslie Kim and Daniel Kim, to the isolated site on a motorboat piloted by Durant’s associate, Craig Burns. Upon arrival, they meet Durant and the facility’s close-knit crew: shark trainers Trent Slater, Mike Shutello, and Josh Hooper, along with computer technician Aaron Ellroy. In a blunt demonstration, Durant throws Aaron Ellroy into the water and lets the sharks close in, only to halt their attack at the last moment with a device that drives them away in formation. Durant explains that he has been genetically altering the brains of five bull sharks to make them smarter, and Misty’s worry deepens as she processes this unsettling capability.
In the wet lab, Durant tells Misty that she was invited because the alpha shark, Bella, has been behaving strangely. Misty suspects Bella’s odd behavior signals pregnancy, a possibility that Durant can’t reconcile because the tests never showed it. Bella is brought into the lab, and Josh Hooper collects a saliva sample while Bella lunges at him, narrowly missing his arm. At the same time, disaster unfolds on the surface: a second shark knocks the motorboat into an electric control box, causing a catastrophic explosion that damages several systems inside the facility. The sharks flood the building, and Craig is killed in the chaos. Misty realizes that Bella has given birth to a brood of pups that have already attacked Craig, a shocking turn that foreshadows the danger to everyone inside.
Mike attempts to reach the surface, but a shark knocks him unconscious; Trent bravely pulls him to safety, and Misty manages to revive him. As Mike leans over to grab his mask, a shark beheads him, and the flooding intensifies. With the water level rising and pressure unstable, the survivors are scattered: Leslie is knocked unconscious, while Durant and Daniel find themselves alone together, and Misty and Trent end up side by side, as do Aaron and Josh. Trent reveals a troubling detail: Durant has been using a genetics-altering liquid on himself to boost intellect, driven by a fear that technology will someday rule the world. Durant soon reunites with Misty and Trent, and the trio moves toward an air ventilation shaft that will take them to the surface.
Meanwhile, Aaron and Josh stumble into one of the sleeping quarters. The shark pups track them down, killing Josh, while Aaron escapes on an air mattress. Misty, Trent, and Durant reach the shaft, but Durant locks Misty out of the room when she tries to search for other survivors, sparking tension with Trent. The trio manages to reach the surface, while Daniel finds Leslie behind a door and is forced to watch as the pups kill her in front of him. Daniel is then found by Aaron, who has been attacked by pups and loses his mattress, and the two of them scramble to find a way out.
Misty uses a blow torch to breach the room containing the ventilation shaft, where she reunites with Aaron and Daniel. The shark pups find them and Misty distracts the predators long enough for Aaron and Daniel to start climbing toward the surface. Water continues to rise around them, enabling the pups to reach Daniel and bisect him as Aaron finally breaks the surface. Misty, with scuba gear she salvages from Trent’s quarters, makes it to the surface and is rejoined by the others.
Realizing the facility is sinking and that they cannot stay, the group makes a break for a motorboat. Aaron is dragged underwater by one of the sharks, and Durant is confronted by Bella, who seems to overwhelm him before unexpectedly leaping up to kill him. Misty and Trent reach the motorboat and defend themselves with two flare guns, killing one of the sharks in the process. Aaron stows aboard, explaining that he somehow escaped another attack. Misty urges the crew not to let the sharks roam the open ocean, and Trent—who had anticipated this exact scenario—activates the facility’s self-destruct, blowing it up and killing many of the sharks offscreen. The trio escapes on the motorboat, but the danger isn’t over: Bella and a sizeable brood appear to have survived the explosion and are now free in the open sea.
