A group of friends heads to the water for a day of wakeboarding, only to be stalked and devoured by a terrifying two-headed shark. At the same time, a Semester at Sea voyage aboard the Sea King unfolds under the leadership of Professor Babish and his wife Anne Babish. A massive shark collides with the vessel and becomes lodged in the propeller, tearing into the hull and forcing the ship to take on water. The danger escalates as the two-headed predator rips through the ship’s defenses, severing the radio antenna and cutting off any chance for urgent assistance from Laura, who co-captains the Sea King.
With help scarce, the group discovers a deserted atoll nearby. Laura steers the Sea King toward it, while Professor Babish gathers the students—Kate, Paul, Laura, Liza, Cole, Lyndsey, Ethan, Jamie, Ryan, Kirsten, Alex, Kristen, Kirk, Haley, Jeff, Alison, and Dana—onto the atoll via dinghy.
Inside the wrecked ship, the danger remains acute. Laura dives to repair the hull but is tragically torn apart by the two-headed shark. Outside, the students split tasks—some scavenge for scrap metal while Kirsten tells Kate she fears the water. After a moment of uneasy calm, Kirk, Haley, and Alison decide to swim and are seized by the predator.
Back on the atoll, the group discovers two small speedboats. An earthquake rattles the area, injuring Professor Babish, who sustains a deep leg cut. Jeff and Mike take the professor back to the Sea King while Jeff and Mike try to return to the atoll. They find Laura’s severed hand in the surf and are then overwhelmed by the shark, losing their lives in the water.
As Kate and Paul repair one boat and Cole sources a gasoline tank, Cole decides to move out with Ryan, Jamie, and Alex in the other. The boats race toward safety, unaware of the predator’s relentless pursuit. The two-headed shark closes in on Cole’s craft, causing Ryan to fall overboard and be eaten, prompting the rest to rally.
Kate, Paul, and Dana rescue Cole and return to the atoll, followed by Professor Babish, Anne Babish, Han, and Dikilla on the dinghy. Faced with annihilation, the survivors hatch a plan: rig a generator to metal poles and drop them into the water to lure the creature away from the Sea King while Kate and Cole repair the hull.
The gambit buys time, but tragedy still strikes. The shark attacks the poles, dragging Han and Dikilla into the sea, where they are devoured, and the Sea King closes in on the island. Kate manages to fix the ship, yet Cole abandons her to flee, leaving Jamie and Alex to be eaten as a consequence of the chaos. Kate and Cole reunite and reach the atoll just as the Sea King sinks, its distress signal a final, desperate call for help.
A fresh earthquake shakes the area and the atoll itself begins to crumble, leaving the last survivors scrambling for any shelter. Kristen and Dana become separated and are eaten, and so too do Professor Babish and Anne Babish as the landscape collapses around them. A small tsunami follows, and the atoll sinks beneath the waves, leaving only Kate and Paul clinging to a fragment of land.
In the final moments, Kate and Paul discover the remaining gasoline tank and use it to lure the shark toward a makeshift trap. Kate makes a bold move to stab the two-headed predator, while Kirsten’s earlier attempt to ignite the tank with a lighter ends in tragedy as she is eaten. With the island slipping away, the two survivors find a floating refuge and, just as the shark closes in, they hop aboard a remaining boat. The waterlit danger culminates in a dramatic explosion that kills the beast, and a helicopter arrives to lift Kate and Paul to safety—the lone survivors of a group once numbering twenty-three.
