 It s the start of the summer term and there s a blizzard raging outside Cackle s Academy. The girls are all desperately trying to stay warm and Miss Drill tries to get the blood flowing by having the girls jog around the school. Meanwhile, Mrs. Tapioca is having problems with a GIANT mouse in the kitchen. Amidst all the chaos, who should arrive seeking shelter from the weather, but Chief Wizard Hellibore, Algernon Rowan-Webb and three wizard apprentices Barry Dragonsbane, Gary Grailquest and Merlin Flagstaff. Algernon is in a particularly mischievous mood and in order to cheer everybody up, pronounces the day to be an upside-down day. Everything is turned on its head; teachers and pupils change places and there are games instead of lessons. Fenella and Griselda teach a Potions  class, while the others play games in the Great Hall. The grand finale of the day is a shape-changing competition between The Chief Wizard and Algernon, but things go terribly wrong when Algernon changes himself into a mouse and is threatened by The Chief Wizard in the shape of an owl. He escapes to the kitchen where he is in equal danger from Mrs. Tapioca s cat, Lightning. Mildred and Merlin save the day and as a thank-you, Algernon invites Mildred and her class to his riverside retreat where he has decided to retire.