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The movie opens with a family of four—Ben, Julia, Tom, and Cassie—who set out on their yacht to celebrate young Ben's 13th birthday. Their adventure begins cheerfully, with Tom, an oceanographer, teaching Ben about fishing, while Julia, a doctor, and Cassie relax under the sun. However, the calm is shattered when Julia decides to take a swim.
As she plunges into the azure waters, an unsettling feeling washes over her. Unable to determine the source of her discomfort, Julia is suddenly swept further away from the boat by an unpredictable current. After Tom rescues her from drowning, he warns the family about the treacherous waters, unaware that this day will set off a chain of events that will test their survival instincts.