Cassie's family is on a beach clean-up expedition in Oregon, and this time Cassie's best friend Rosalie has joined them. The two girls are ecstatic about the vacation and Cassie feels ready to tell Rosalie about her bizarre adventures . . . until Rosalie ditches her to play volleyball with a new friend.

Hurt and betrayed, Cassie wanders off down the beach and discovers a real mermaid named Galena! Before long, the sneaky sea creature magically tricks Cassie into switching bodies. Missing her family and her legs, Cassie soon learns that life under the sea isn't so great, especially when she's jailed for all of Galena's crimes. If she can't convince Galena's suspicious mermaid friends to help her switch back, Cassie may be stuck as a convicted mermaid for the rest of her life!