13-year-old Bethany Hamilton (AnnaSophia Robb) is a champion surfer living in Hawaii with her parents and 2 older brothers, all of whom love to surf. The family is close and share a strong Christian faith. On a regular day in the water with her best friend's family, Bethany's arm is bitten off by a shark. She remains surprisingly calm and strong while being rushed to the hospital, where her own father is coincidentally having minor knee surgery.
The recovery is slower than Bethany would like, and she only cares about getting back into the water to train for the upcoming regional competition, for which she and her best friend have recently been sponsored by a major corporation. She trains hard but, not being able to get a good grip on her board while being hit by a wave, doesn't do well in the competition and gives up, discouraged for the first time.
A church trip to bring aid to the tsunami-devastated area in southeast Asia helps Bethany gain perspective, as does a growing number of letters from kids around the world who tell her that she has inspired them to conquer their own challenges. She commits to training harder to compete in the upcoming nationals, and her father (Dennis Quaid) gives her a modified surfboard that has a grip handle on it. Her brothers film her practices, helping her see how to adapt her form and shift her balance to compensate for the loss of her arm.
Bethany places only 5th in nationals but gains the respect of a formerly spiteful competitor. The next year, she wins the national, and continues to compete, eventually turning pro.