Nicola and Lawrie Marlow, the youngest in their large family, are going to boarding school for the first time. On the train they meet another new girl, Thalia Keith, known as Tim; who is the niece of the headmistress. Nicola stops the train by accident after dropping her penknife; which gets her into trouble. Once at school things don't go as well as the twins hoped; life is complicated by form room disputes, mishaps in guides; and because their eldest sister is Head Girl.