The MS Poseidon, a luxury cruise ship, is on a year-end transatlantic crossing to New York. Jennifer (Emmy Rossum) is with her fiance, Christian (Mike Vogel), and they consider how to tell Jennifer's father, former New York City Mayor Robert Ramsey (Kurt Russell), about their engagement. In the galley, stowaway Elena (Mía Maestro) searches for her friend, a waiter named Valentin (Freddy Rodriguez). Architect Richard Nelson (Richard Dreyfuss) is making a phone call to his estranged lover. In the grand ballroom, Captain Michael Bradford (Andre Braugher) delivers a New Year's Eve speech before introducing the singer, Gloria (Fergie). Dylan (Josh Lucas), a professional gambler and retired Naval sailor, and Robert play Texas Hold'Em with Lucky Larry (Kevin Dillon). Jennifer, Christian and Elena are partying in the nightclub. Richard is with his friends, talking about his heartbreak. As Dylan walks with his winnings, he bumps into Connor (Jimmy Bennett). Maggie (Jacinda Barrett), Connor's mother, arrives and chats with the gambler.On the bridge, officers see a 150-foot rogue wave coming towards them, while everyone celebrates the New Year countdown. On the promenade deck, Richard, who is contemplating suicide, sees the wave and retreats inside. Though the crew on the bridge sounds warning alarms and turns hard to starboard to meet the wave head-on, the Poseidon turns too slowly. It is hit broad-side and capsizes. Passengers are swept overboard, fall to their deaths in overturned high-ceiling areas or are crushed to death by falling debris. In the galley, crew are incinerated by flash fires caused by rupturing gas lines.In the ballroom, Robert, Dylan, Richard and Maggie wake up, realizing that the ship is now upside down. Robert sees Connor on the piano, which is now on the ceiling. Robert and some officers rescue Connor. In the nightclub, Jennifer looks for Christian, who has been pinned by a fallen light. They and Elena are the only ones to avoid being killed by the combined dangers of the sprinklers activating and numerous broken wires and lights electrifying the water.Captain Bradford reassures the passengers by telling them that a set of GPS beacons were launched during the collision and that help is on its way, therefore they should stay put. Some passengers hesitate. Dylan disagrees and concludes that the bow Thrusters of the ship are the key to survival. He is joined by Robert, Richard, Connor, Maggie and Valentin. The group makes their way through the kitchens to a service elevator. Using a table to bridge the space across the shaft, the group pries open an opposing set of elevator doors. Everyone survives until the table drops as Richard is being pulled up by Dylan. The service elevator, severely damaged by the collision, begins to plummet towards them, and Richard is forced to shake the crewman off. Valentin falls down the elevator shaft and is killed immediately by the fall, and the elevator crashes down moments after Richard is pulled through the doors. The group manages to force the elevator doors shut just before a gas explosion set off by the falling elevator reaches them.Meanwhile, in the nightclub, Elena and Jennifer rescue Christian, with help from Lucky Larry. Robert and Jennifer reunite with the other survivors. The group reaches the lobby and uses a damaged elevator as a bridge. As Lucky Larry takes his turn to cross, an engine smashes its way through the lobby floor above and kills him. Large amounts of leaking fuel follow through the opening in the floor and is ignited. Dylan finds a fire hose and jumps into the flaming water, making a zip line that the others use to cross.In the ballroom, the windows at last give way, the room floods and the surviving passengers, Captain, Gloria and the crew inside are drowned to their deaths. As the surviving group tries to escape from the rising water, they hear all the people from the main hall screaming for help. they go into an air vent which leads to the ballast tanks. Elena struggles to continue moving through the air vent due to her claustrophobia. Robert tries to smash the cover of the vent, but fails, and calls Connor to assist him in the hopes that Connor's smaller fingers will be able to undo the vent screws. Elena successfully gets inside the vent and offers her necklace to be used as a screwdriver.The cover opens and the group climbs into the ship's ballast tank, where they locate a hatch.Robert and Dylan realize that only great pressure can open the hatch, and they flood the chamber. The hatch opens, and they swim through to the next chamber. Elena's dress becomes caught as she swims through, and she hits her head on a sharp beam and loses consciousness. Richard turns back to rescue her, followed by Robert, Dylan and Christian, and takes the unconscious Elena back where Robert tries to revive her, but she dies from her injury without regaining consciousness.The group finds themselves in the galley and see the bow section is flooded, telling them the obvious: the Poseidon is sinking by the bow, and fast. While finding a new exit, an explosion in the engine room causes the bow to rise out of the water and forces the stern down. All of the water in the bow rushes towards the stern, and the stern of the Poseidon sinks underwater. Robert, Christian, Jennifer and Richard go first, leaving Dylan, Maggie and Connor behind.Maggie and Dylan find Connor trapped. He is eventually saved by Dylan and the three reunite with the group. In the engine room, Richard opens a cover and the bow Thrusters, still trying to turn the ship to starboard, begin blasting air into the room. Knowing that the control room for the propellers is now below water, Christian and Robert debate who should go, knowing the one may not come back. Christian decides he will, only to find Robert has already gone.Underwater, Robert finds the off switch broken, but manages to hit the reverse button instead just before he drowns. The propellers briefly stop, then begin turning the other way, and Dylan finds a nitrogen tank and throws it into the propellers. The ensuing explosion destroys the motors for one of the three bow Thrusters. After climbing through the propeller tube, they jump overboard and into an inflatable raft. The survivors start rowing for their lives, as a wave is thrown up as the Poseidon rights herself again, pushing the survivors away from the ship.The Poseidon is too badly damaged to remain afloat anymore, however, and, with her bow pointing high into the night sky, she sinks by the stern in the foaming waters as the handful of survivors looks on. Finding a flare gun in the raft, Dylan fires it, and they wait for rescue. Sometime later, a group of helicopters finds the raft and moves in to pick up the survivors. Just before the credits roll, at least three ships are shown to be racing towards where the Poseidon sank.