It's New Year's Eve and the party is in full swing aboard a luxury cruise ship in the North Atlantic. Suddenly, a massive wave strikes the liner, pitching the ship to port then rolling it completely upside down and starting a desperate fight to survive, to escape the capsized and sinking Poseidon. Passengers and crew are thrown into free fall, crushed by debris or dragged into the sea as water bursts in through shattered windows. Supports collapse, broken gas lines ignite flash fires and lights fail, leaving vast sections of the ship in darkness and chaos. In the aftermath, a few hundred survivors are left to huddle in the still-intact main ballroom, now resting below the waterline. Determined to fight their way to the surface, they must forge a path together through layers of wreckage as the ship begins to sink.