Sarah, 12, lives on a boat with her parents, who research whale sharks. Her only friend is a whale shark named Jack, who Sarah rescued when he was a pup. But following a tragic accident, mum and Sarah move to the coastal town of Exmouth. Desperate to return to her old life, Sarah pegs all her hopes on Jack, certain that when his migration brings him back to Exmouth, mum will take her out on the boat again. But Jack fails to return, so Sarah embarks on a risky mission to find him.
