In Circus Debut, Isabel Blossoms and her somewhat perplexing Grandma are on an adventure-filled trip to St. Cloud, Minnesota, where they plan to attend a popular play entitled Polly of the Circus. While on a tour of the St. Germain Theatre, Isabel befriends Sylvia who acts the part of Polly in the play. Isabel soon learns that Sylvia and several of the other cast members are entangled in numerous mysterious events. While solving the mysteries of the circus, Isabel and Grandma become part of the play as they participate in the unusual, thrill-packed circus parade. Isabel's curiosity and Grandma's witches' chants and magic herb throwing lead to interesting discoveries which resolve the mysteries for the playgoers and performers.