The luxury cruise ship MS Poseidon embarks on a glamorous year-end journey to New York. Onboard, Jennifer (Emmy Rossum) is excitedly considering how to break the news of her engagement to her father, former New York City Mayor Robert Ramsey (Kurt Russell), while she enjoys the company of her fiancé Christian. Meanwhile, stowaway Elena searches for her friend, a waiter named Valentin. The ship's architect, Richard Nelson (Richard Dreyfuss), is preoccupied with thoughts of his estranged lover.
As the grand New Year’s Eve celebration unfolds, Captain Michael Bradford speaks to the crowd, introducing singer Gloria while a group of characters indulge in their personal dramas. In the midst of the merriment, a catastrophic rogue wave strikes the ship, leading to chaos and tragedy. Passengers are thrown about, many meeting their fate as the vessel capsizes in a catastrophic disaster. Flash fires erupt in the kitchen, and crew members are tragically caught in the chaos.
After waking up in the upside-down ballroom, Robert, Dylan, Richard, and Maggie scramble to save young Connor, who has found himself in a perilous position. As the ship continues its descent into chaos, Jennifer and Christian manage to evade immediate danger but are separated in the frenzy. Captain Bradford’s reassurances fall on deaf ears as Dylan realizes that survival hinges on a different plan – accessing the bow thrusters. A desperate journey through the ship leads the group into perilous situations: torn elevator wires, fiery floods, and ultimately a race against time.
As they traverse through air vents and flooded chambers, the reality of their situation becomes increasingly grim. Tensions rise as claustrophobia sets in for Elena. Helping one another, they make sacrifices, but tragedy strikes when Elena succumbs to her injuries sustained while attempting to navigate their dire escape. As explosions ravage the ship, the survivors realize they must push forward, battling against both the elements and the sinking ship.
Emerging into the bowels of the Poseidon, the group desperately searches for a way out, facing multiple life-threatening challenges. Ultimately, Robert takes a heroic plunge into the depths to save the ship from total destruction, resulting in an explosion that damages the ship further. They manage to escape just in time, finding solace in a makeshift raft as the mighty vessel succumbs to the sea, finally sinking amidst the turmoil.
With the wreckage of the Poseidon behind them and hope on the horizon, Dylan fires a flare, signaling for rescue. As helicopters swarm to retrieve the survivors, the camera captures the vast ocean where the ship once sailed, hinting at the chilling echo of the ship's tragic end.
