Carly, a nine-year-old British Columbian girl, adores swimming and diving, spends summers with her family at a cabin on the Gulf Islands, and she's also a member of Dolphins Diving Club, a girls' club at the local town swimming pool.

One day, Carly is given permission to invite her best friend, a girl named Montana, to stay at the family cabin.

Carly, who is often at odds with her bickering siblings, feels a strong bond with Montana that the girls both share over swimming.

When Montana breaks her spine and faces a life of rehabilitation and potential paralysis, Carly is angry and confused and struggles with her feelings of guilt.

The book also explores her relationship with her step-father and two older step-siblings, in juxtaposition with the earlier death of her biological father.