Every summer, Gideon and his younger sister Audrey build a sandcastle together. But this summer, everything changes. Gideon decides to build the most spectacular sandcastle anyone on the beach has ever seen. And he's going to do it on his own--without any help from his sister.

But much to his surprise, Gideon discovers that building together is more fun and that everyone has their own unique talent when it comes to creativity and imagination, even Audrey.