In a remote underwater facility, doctors Susan McCallister and Jim Whitlock are conducting research on
mako sharks to help in the re-activation of dormant human brain cells like those found in
Alzheimer's patients. After one of the sharks escapes the facility and attempts to attack a boat full of young adults, financial backers send corporate executive Russell Franklin to investigate the facility. Susan and Jim prove their research is working by testing a certain
protein complex that was removed from the brain tissue of their largest shark, which suddenly bites off Jim's arm upon awakening in the laboratory. Tower operator Brenda Kerns calls a helicopter that flies through heavy rain and strong winds to evacuate Jim. As Jim is being airlifted on a stretcher, the cable jams and he is dropped into the shark pen. The largest shark grabs the stretcher, which is still attached to the cable, and uses it to pull the helicopter into the tower; the resulting explosions kill Brenda and the helicopter pilots while severely damaging the facility. Susan, Russell, shark wrangler Carter Blake, marine biologist Janice Higgins and engineer Tom Scoggins witness the shark smash Jim's stretcher against the laboratory's main window and shatter it, drowning Jim and flooding the facility. They go to the facility's
wet porch, where they plan to take a
submersible to the surface. Susan confesses that she and Jim
genetically engineered the sharks to increase their brain size as they were not naturally large enough to harvest sufficient amounts of the protein complex, breaking protocol and making the sharks much smarter and deadlier. In the flooded kitchen, cook Sherman "Preacher" Dudley hides in the oven from a stray shark that eats his pet parrot. The shark accidentally causes a gas leak from the oven while trying to break into it, and Preacher sneaks away and kills it by setting off an explosion with his lighter. When the group reaches the wet porch, they discover that the submersible has been damaged and is unsuitable for use. While delivering a monologue emphasizing the need for group unity, Russell is dragged into the submersible pool by the largest shark and devoured. The remaining crew opt to climb up the elevator shaft at the risk of
destabilizing the pool. As they climb, explosive tremors cause the ladder to break. Janice loses her grip and falls into the water. Despite Carter's attempt to save her, a shark drags her under and devours her. The rest of the group move on and encounter Preacher. Carter and Tom go to the flooded laboratory to activate a control panel that drains a stairway to the surface, while Susan heads to her room to collect her research materials. Carter and Tom reach the control panel but the largest shark storms in, rips Tom apart, and damages the controls. In her room, Susan encounters another shark and electrocutes it with a power cable, accepting that she had to destroy her research in the process. Carter, Susan, and Preacher regroup and go to a
decompression chamber. They swim to the surface while using oxygen tanks to bait the last shark into an attack. Upon reaching the surface, Preacher is grabbed by the shark and suffers injuries to his leg, but the shark releases him when he stabs it in the eye with his crucifix necklace. Carter realizes that the sharks have been manipulating them to methodically flood the facility so that they can ram their way through the fences at the surface and escape into the world. To keep the final shark from escaping, the three make a plan to blow it up by shooting it with a harpoon and connecting the harpoon's wire to a battery, sending a massive electric current to an explosive charge in the harpoon. Feeling guilty over causing the entire situation, Susan uses herself as bait without telling the others, cutting her hand before diving into the water. She distracts the shark with her blood, but is unable to get out of the water in time and is devoured, despite Carter's efforts to save her. Carter grabs onto the shark's dorsal fin and Preacher shoots it with the harpoon, which accidentally pierces Carter's thigh and pins him to the shark. Carter nevertheless orders Preacher to connect the wire to the battery, and manages to free himself mere seconds before the shark rams through the fence, where it explodes as it tries to escape. Carter resurfaces and swims to shore, reuniting with Preacher. Moments later, as they sit on the edge, they see a rescue boat approaching the sinking facility. Preacher wryly advises Carter to stop dangling his legs in the water, which he immediately does. ==Cast==