Mildred Hubble is distraught. On her first day back at Miss Cackle’s Academy for Witches, her beloved but useless cat, Tabby, is banished to the kitchen to catch the mice. Mildred’s class is going to spend a week by the sea. She can’t possibly go without Tabby! So she hides him away – but trying to keep him hidden from Miss Hardbroom leads Mildred from one disaster to another, and soon she really is all at sea.