The children's story starts with the twins Pat and Isabel O'Sullivan looking forward to enjoying their first summer term at St. Clare's. Their mother is happy to see them looking forward to school. One day, they go to play tennis with a friend, and meet a girl who has mumps. The twins, to their dismay, are put in quarantine, and are not allowed to go back to school at the beginning of the term.
When they arrive back the following week, they are heartily welcomed by their friends. Five new girls have joined their form. There is an American girl called Sadie, who is obviously quite rich and elegant. The twins discover that their cousin Alison has already made friends with her. There is a wild-looking girl called Carlotta, who is half Spanish. There is a naughty but very likeable girl Bobby (Roberta), who quickly becomes friends with Janet. There is another girl called Prudence, who is quite pretty but has no sense of humour. The last new girl is Pam, who is very hard working but also very shy.