Garden Irene McGeeny has a secret. One that she's not supposed to tell anybody. It is that her family lives in an underground castle! She tries to keep her secret, but she mistakenly blurts it out during class—the rest of the students don't believe her. Peter, who becomes an unlikely friend, tries to prove her wrong, which leads Garden to do the unthinkable: take him to her home! Realizing the trouble Garden would be in if her parents find out or if the rest of the kids at school know the truth, Garden and Peter try to keep the secret a secret. Will the rest of the kids find out? Will the truth of Garden's home become known by all?