The two girl twins, Patricia and Isabel O'Sullivan, having just finished school at the elite school called Redroofs, are expected to move on to senior school. While most of their friends at their old school, two of them Mary and Frances Waters, are moving to the equally elite Ringmere, the twins' parents are reluctant to send them to an expensive school as they are afraid the twins might become spoilt and snobbish. Furious at their parents' refusal to send them to the school of their choice, the twins are determined to be as difficult as possible at St. Clare's.

The twins miss their favourite sports of field hockey and tennis because only lacrosse is played at St Clare's. However, Pat turns out to be quite good at lacrosse. She is selected by sports captain Belinda Towers despite having defied her earlier.

The twins soon make good friends with the other girls and play pranks on others in the school.