新年的钟声即将响起，欢快喜庆的气氛弥漫在正孤独航行于北大西洋上的海神号中。这是一艘拥有800个房间、13层楼高、载客量超过2000人的超豪华邮轮。在船长麦克•布莱福德（Andre Braugher 饰）的组织下，达官显贵、社会名流纷纷云集在豪华明亮的大厅，伴着美食佳酒、轻歌曼舞一同庆祝新年的到来。与此同时，大西洋底的地壳发生剧烈震动，继而掀起足以摧毁万物的滔天巨浪。在自然面前，再坚固的船只也需俯首称臣。海神号被巨浪掀翻，无数人丢掉性命。面对死神的威胁，侥幸逃过第一波劫难的人们开始想尽办法逃生。在茫茫无所依靠的大海中央，他们的命运终将如何？ 本片翻拍自1972年经典灾难片《海神号遇险记》（The Poseidon Adventure）。
In the northern Atlantic Ocean, the ocean liner Poseidon, named for the Greek god of water, is sailing through the ocean. On one of its thirteen decks, we see Dylan Johns (Josh Lucas) running for an evening workout, stopping when he reaches the top to enjoy the last sunset of 2005. Meanwhile, in one of the 800 cabins, Jen Ramsey (Emmy Rossum) is canoodling with her boyfriend/fiancée Christian (Mike Vogel) when they are interrupted by Jen's father, former firefighter/mayor of New York Robert Ramsey (Kurt Russell). Christian immediately starts sucking up to the older man, who tells Christian, "Stop calling me sir....it makes me feel old." He also implies that the kids are having sex, which Jen flatly denies before stomping off to her room of the large suite. Down below in the kitchen, an attractive Spanish girl named Elena (Mia Maestro), is hunting for her friend Marco Valentin. She finds Marco (Freddy Rodriguez), a waiter on the ship, and we quickly learn that he is smuggling her across from Europe so she can be with her sick brother in New York. He gives her some food from a nearby plate, but he tells her that she has to stay in the cabin. Elsewhere, gay architect Richard Nelson (Richard Dreyfus) calling his boyfriend, but he only gets the answering machine, and we can tell that his lover recently broke up with him.
That night, Captain Michael Bradford (Andre Braugher) welcomes everyone to his ship. He tells them a little about its namesake and then introduces musical act Gloria (Stacy Ferguson). While she performs we go up a few decks, Dylan and Ramsey are playing a game of Texas Holdem with two others, generic guy five and Lucky Larry (Kevin Dillon). Jen and Christian come up to say hello to Ramsey before heading over to the nightclub as he's nearing a fifty thousand dollar betting mark with Dylan (Larry and guy five folded). Ramsey asks Jen to button up her dress to cover her breasts, and as she does says loudly for all about his hand, a pair of fives . Ramsey looks up at Dylan, who is grinning evilly and saying, "All in." Over at another table, we see Nelson telling some of his friends the story of his breakup and his ex's quick turnaround while he orders a five thousand dollar bottle of wine for the table just before excusing himself to get some air. Meanwhile, we see Dylan walking away with his winnings when he bumps into a kid playing with his PSP. The kid is Connor James (Jimmy Bennet) and he takes an immediate liking to Dylan, as does his mom Maggie (Jacinda Barret) who quickly comes up looking for him. Meanwhile, we see Jen and Chris in the nightclub partying like crazy as Elena picks her way through the crowed lower decks.
On the bridge the officer of the watch gets a feeling that "Something's off," when the instruments suddenly detect a HUGE 150-foot tall, tidal wave coming towards the ship (Nelson, who was about to jump overboard, also sees this and it ironically makes him rethink the suicide). The officer orders a hard to starboard in order to meet the wave head on, but it is too late. The wave hits the starboard side of the ship, sweeping dozens of people overboard (some of whom were in the on-deck pool), crushing others, flash-frying some, electrifying a few, and in general causing all sorts of hellish chaos. The Poseidon completely turns over, upside down and within seconds, all power on the ship goes off and the lights fail, plunging the capsized ship into darkness.
