A ghostly figure, a walking suit of armor, and a gargoyle in the prompter's box confront Jean and Louise Dana and their Starhurst classmates as they rehearse for a musical at Mozart Hall.

The owner, a retired singer, and the watchman assure the performers there is nothing to worry about. The students wonder whether their imaginations are playing tricks on them. Or are spooky things actually happening each time they visit the opera house?

A contest is held for an original song to be used in the play. The Danas' music is stolen but they make a copy and submit it. When their song is chosen as one of the winners, a listener says she has heard it before! Louise and Jean face danger and intrigue, but they work hard to solve several mysteries and to win out over their enemies.