During World War II, the all-girls St. Swithins school is evacuated from London to avoid the Blitz.

Due to an administrative error, the female students and staff are relocated to the Nutborne Boys School, whose headmaster, Wetherby Pond, is alarmed. He and St. Swithins headmistress Muriel Whitchurch agree to keep their mixed-sex campus a secret to avoid disturbing the parents, but they find it hard to deceive visiting inspectors.