Challenges, friendship, and the spirit of competition will lead two friends to push their limits and touch the very edges of their dreams.

As she does every year, Nine meets up with her best friend Maï on an island off the coast of Brittany. Together, they practice open-water swimming and challenge each other to push past their limits. But the summer of their seventeenth birthday is different. Something happened earlier that year, and Nine can’t swim the way she wants. The book alternates between a series of flashbacks to their friendship, developing over the summers, and the challenge set by Maï this year, switching back and forth between their points of view, and revealing what befell Nine as well as both of their reactions.
How far will sensible but brave Nine follow independent and unpredictable Maï?