Since her twin brother, Eddie, drowned five years ago, sixteen-year-old Elsie Main has tried to remember what really happened that fateful day on the beach. One minute Eddie was there, and the next he was gone. Seventeen-year-old Tay McKenzie is a cute and mysterious boy that Elsie meets in her favorite boathouse hangout. When Tay introduces Elsie to the world of freediving, she vows to find the answers she seeks at the bottom of the sea.