When a mysterious letter arrives from a woman claiming to know of a Method to make Betsy Bow-Linnet's piano playing 'stupendous', Betsy jumps at the opportunity. There's just one small condition: she must keep the Method a secret.

When Betsy's dad agrees to bake the cakes for a Royal Gala he has no idea how big a job it is. Betsy knows the only way he'll get it done is with the help of her forty-four mice - just as long as she can keep them away from prying eyes...