When a poor street mouse named Angus accidentally wanders onto the stage of the opera, he scares the soprano so much that she sings a record-breaking high note. Word gets out, and soon the soprano, Minnie McGraw, becomes world famous, and goes on tour with Angus as her secret. But things go wrong when Angus becomes famous in his own right and Minnie grows jealous. Minnie and Carlo, the scheming tenor, come up with a plan to mousenap Angus! Trapped in a cage backstage, Angus must escape in time to save the opera company from ruin.