Maria Bennett, her husband Henry Bennett, and their three sons Lucas, Thomas, and Simon go on a Christmas holiday in 2004 to Khao Lak, Thailand. Arriving on Christmas Eve, they settle in and begin to enjoy the brand new Orchid Beach Resort. Two days later on Boxing Day, the massive 2004 tsunami inundates the area.
Maria and Lucas eventually emerge from the swirling water and find one another, with Maria having sustained serious injuries to her leg and chest. They help a toddler, Daniel, from the wreckage and are soon found by locals who transfer them to a local hospital in the city of Takua Pa. Daniel is separated from them during the journey. At the hospital, Maria encourages Lucas to help others find their family members while she goes into surgery for her chest injuries.
Meanwhile, Henry, Thomas and Simon have also survived and are together. Henry leaves the two boys with another family who are taken to the mountains for safe shelter by local relief crews while he stays behind to search for Maria and Lucas. While out looking, injured and alone, he is picked up by a passer-by and driven to a nearby bus shelter to wait out with other survivors. Communication facilities are scarce, but eventually a tourist named Karl, who has also been separated from his family, lends Henry his cell phone to contact his relatives. Henry promises Maria’s father he will look everywhere for his family and that he will find them. Karl tells the group his wife had left him a note confirming they were at the beach, their location when the wave hit. He volunteers to accompany Henry to look for Maria and Lucas and his own family, too.
While Maria is in surgery, her medical chart is mixed-up with another patient who has died. Lucas returns to find his mother’s bed empty and is then taken to a tent where children without families are kept safe. The mistake is discovered when Lucas cannot identify any of the dead woman’s jewelry and he is subsequently reunited with his mother, who has been moved to a private room in the ICU. In the hospital, while he waits, Lucas finds Daniel, who has been reunited with his father.
Henry and Karl search for their families in various places before they arrive at the hospital, where Henry is given five minutes to look. Karl gives him a piece of paper with his family members’ names on. The vehicle carrying Thomas and Simon also stops outside the hospital, and the boys get off so Simon can urinate. From a distance, Lucas recognizes his father, and while searching him out in the chaotic crowd outside, Lucas’ brothers spot him and they reunite. Henry finds the three of them together. He learns that Maria is in the hospital, ready to undergo more surgery for her leg, which she survives. Flashbacks reveal how she came to be badly injured and how she surfaced the water. While in surgery, Lucas tells his father he has something really important to tell Maria.
The following day, the family boards an ambulance airplane to Singapore so Maria may receive further medical treatment, arranged by their insurance company. On the plane, Lucas tells his mother he knows Daniel is safe. Maria cries as she looks out the window at the chaos left behind.