Darrell Rivers and her friends have finally reached the Fifth Form at Malory Towers, and this term brings the most exciting event of the year: the girls get to write, direct, and star in their very own Christmas pantomime! Darrell is absolutely thrilled to be chosen as the head scriptwriter for their grand production of Cinderella. But the theatrical magic is quickly overshadowed by intense backstage drama. The newly appointed head of the form, Moira, is acting like a brutal dictator, bossing everyone around and making bitter enemies out of the cast. Meanwhile, a smug new girl named Maureen arrives from a rival school, determined to steal the spotlight and drive everyone crazy.

The tension completely boils over when a series of cruel, anonymous "poison pen" letters begin arriving, threatening to destroy the play and tear the form apart. With morale at rock bottom and the grand performance looming, Darrell must uncover the identity of the phantom letter-writer. Packed with spectacular stage performances, bitter rivalries, and a shocking unmasking, the Fifth Form's pantomime will be a show no one will ever forget!