Enredo

In 2004, Doctor Maria Bennett (Naomi Watts), her husband Henry (Ewan McGregor), and their three sons Lucas (Tom Holland), Thomas (Samuel Joslin), and Simon (Oaklee Pendergast) go on a Christmas holiday to Khao Lak, Thailand. Arriving on Christmas Eve, they settle in and begin to enjoy the brand-new Orchid Beach Resort. Henry works in Japan as a contractor, and it seems like his employers are not keen on extending his contract. This means that Maria might have to go back to work as a physician to keep the family afloat.

The beach resort is right next to the sea. The beach side of the resort is lined with cottages, followed by a pool, and then the main hotel building, which also holds the restaurants. The family had booked a room on the 3rd floor, but when they arrive, they are upgraded to a beach facing cottage. After a blissful night as a family, the next day they undertake some water-based activities, exploring the shoals & corals surrounding the beach.

December 26, started like any other day. The family was in beach wear (very light clothes). The kids were playing table tennis by the pool as Maria headed to the beach. Later, Henry joins the kids in the pool while Maria enjoys a book by the poolside. This is when a massive Indian Ocean tsunami inundates the area. At first the resort loses its electricity supply and Maria notices the wind changing direction suddenly. The birds start flying inland in great numbers and a huge wall of water gushes in from the ocean and hits the resort.

Henry had Thomas and Simon in his arms, but Lucas was separated. Maria is submerged in water and only survives by getting hold of a tree trunk that is not uprooted by the force of the water. Maria sees Lucas drifting away with the water current and goes after to save him. As the water starts to stabilize, a 2nd and bigger wave of water hits the land from the ocean-side.

Maria and Lucas eventually emerge from the swirling water and find one another, with Maria having sustained serious injuries to her leg and chest (the submerged branch of a tree punctures a hole in her chest). Lucas sees that a huge chuck of flesh is just hanging off from the back of Maria's leg. Maria manages to bandage her wound with a piece of cloth that she finds in the debris. Lucas has to support Maria as she is unable to walk on her own.

They help a toddler, Daniel (Johan Sundberg), from the debris. Lucas is livid with Maria for helping Daniel, who says that Lucas would want others to help Thomas and Simon. Lucas yells that Thomas and Simon are dead.

They climb a tree to get out of the water (Lucas finds a can of Coca Cola that the trio share for some energy) and are soon found by locals (an old man who drags Maria to his vehicle and takes her to his village) who dress and transfer them to a 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. She was bleeding internally and was spitting blood and guts out of her mouth.

Meanwhile, Henry and the two younger boys have also survived and are together. Henry leaves Simon and Thomas with another family who are then taken to the mountains for safety by local relief crews. Henry stays behind to search for Maria and Lucas in the rubble. While out looking, injured and alone, he is picked up by a passer-by and driven to a nearby bus station with other survivors.

Communication facilities are scarce, but a European tourist named Karl (Sonke Mohring), who has also been separated from his family, lends Henry his cell phone to contact his relatives in England. Henry promises Maria's father he will look everywhere for his family and that he will find them. Karl volunteers to accompany Henry to look for Maria and Lucas as well as his own family, who were at the beach when the tsunami hit.

While Maria is in surgery, her medical chart is mixed up with a patient named Muriel Barnes, who has died (in the bed next to her). 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 had been moved to a private room in the ICU. While he waits in the hospital, Lucas sees 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 the names of his family members. A 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 for him in the chaotic crowd outside, Lucas's 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.

As the anesthesia puts her to sleep, Maria experiences flashbacks of how she came to be injured and how she surfaced the water. While she is in surgery, Lucas tells Henry 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. A representative from their insurance company, Zurich Insurance, assures them everything will be taken care of as Lucas sees countless people outside the hospital looking through patient lists. On the plane, Lucas tells his mother that Daniel is safe with his father. Maria cries and looks out the window at the chaos left behind as the plane takes off.