Maria Bennett, portrayed by Naomi Watts, along with her husband Henry (Ewan McGregor) and their three sons—Lucas (Tom Holland), Tomas, and Simon—embark on a Christmas holiday at a picturesque resort in Khao Lak, Thailand. However, their dream vacation turns into a nightmare when the catastrophic tsunami of December 26, 2004, wreaks havoc on the coastal area, sweeping them into the chaos. In a heart-wrenching scene, Maria is seen searching for a lost page of a book against a glass window, while the rest of her family is nearby by the pool.
As the family is torn apart, Maria and Lucas find themselves separated from the others. Tragically, Maria sustains severe injuries to her leg. With a mother's bravery, she insists they help a small boy named Daniel, who is also lost and alone. Together, they climb a tree, seeking safety from the impending waves. Thankfully, local villagers discover them and rush them to a hospital, with Maria being transported on a door used as a makeshift stretcher. As a doctor, Maria's concern for her leg grows, and she urges Lucas to keep an eye on it, warning him to alert her if it turns black. She encourages him to assist others in need. Lucas bravely takes on the responsibility of gathering patient names to help locate their families. However, in a tragic mix-up at the hospital, Lucas is led to believe his mother has died. He is eventually taken to a tent with other displaced children, feeling lost and unable to reach out to his grandfather due to memory loss.
Meanwhile, Henry is with Tomas and Simon, ensuring their safety. He instructs Tomas to look after Simon before sending them off in a vehicle full of tourists heading to a safe haven in the mountains. Henry remains determined to search for Maria and Lucas, leading him to a German tourist named Karl, who offers him a phone to contact his father-in-law, Brian, back in the UK. Heartbroken and overwhelmed, Henry struggles to communicate the fear and uncertainty of their situation.
As luck would have it, the family, unbeknownst to each other, has all found refuge in the same general vicinity. In a dramatic turn of events, Henry walks past Maria’s hospital room, and for a fleeting moment, Maria recognizes the shadow of her husband, though her weakened state prevents her from calling out. Lucas, spotting his father in a crowd, quickly loses sight of him, leading to a chaotic yet heartwarming chase. Simon's voice reaches Lucas, and just in time, Tomas and Simon leap from a truck, racing towards their brother. In a tearful reunion, the brothers and Henry come together, with Lucas eagerly telling his father that their mother is in the hospital.
The family finally locates Maria, who is gravely ill and in desperate need of surgery. After a successful operation, a representative from their insurance arrives to arrange for their transport to a hospital in Singapore. In a poignant moment, Lucas shares that Daniel, the boy they saved, has been reunited with his father. As the plane ascends, Maria gazes out the window, witnessing the devastation left behind by the tsunami, and tears fill her eyes, encapsulating both the heartbreak and hope of their incredible journey.
