Henry Bennett (Ewan McGregor), his physician wife Maria (Naomi Watts), and their three sons Lucas (Tom Holland), Tomas (Samuel Joslin), and Simon (Oaklee Pendergrast) 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 the massive 2004 tsunami inundates the area.
Maria and Lucas eventually emerge from the swirling water, with Maria having sustained serious injuries. They find a toddler named Daniel in the wreckage and Maria insists on taking him along despite Lucas' reluctance. They are found by locals, who transfer them to a local hospital where Maria encourages Lucas to help others find their family members at the facility. While he is doing that, Maria goes to surgery for her chest injuries and her medical chart is mixed-up with another patient who had died. Lucas returns to find his mother's bed empty and he is then taken to a tent where children without families are put. The mistake is discovered when Lucas cannot identify any of the dead woman's jewelry and he is subsequently reunited with his mother.
Meanwhile, Henry, Thomas and Simon together have also survived the tsunami, although Henry is injured. The boys are placed on transport under the supervision of a woman to take people to the mountains and Henry remains behind to search for his missing family members. Communication facilities are scarce but eventually a tourist named Karl (Sönke Möhring), who was also separated from his family, lends Henry his cell phone to contact his relatives, and volunteers to accompany Henry to look for Maria and Lucas.
They search for their families in various places before they arrive at the hospital, where Henry sees the woman to whom Thomas and Simon were entrusted. The vehicle carrying Thomas and Simon stops outside the hospital. From a distance, Lucas recognizes his father and while searching him out, Lucas' siblings hear him calling out in frustration. Henry learns that Maria is at that location and they reunite. Maria has surgery, which she survives, and the following day the family boards an ambulance airplane to Singapore for Maria's further medical treatment, arranged by their insurance company.