The play is set in 1947. Dick Tassell is returning as a schoolmaster at Hilary Hall, a boys' school, after five years in the Royal Air Force. Many wartime expedients are still in force, and the staff of the College reconcile themselves to having to share their premises with another school, whose bombed buildings remain in ruins. But by a bureaucratic error, the school to be billeted at Hilary Hall is St Swithins – a girls' school.

After early skirmishing and mutual disdain the Headmaster of Hilary Hall, Godfrey Pond, and the Principal of St Swithins, Miss Whitchurch, try to reach an accommodation to cope with the ensuing problems. Miss Whitchurch establishes an early advantage by getting the men to stop smoking on the premises and to have the dormitories reserved for the girls, with the boys reduced to sleeping in the carpentry room. She is obliged to cooperate with Pond when parents turn up, the girls' expecting netball, the boys', boxing and cricket. It is unthinkable that they should learn that their children are mingling with the opposite sex. By frantic manoeuvring the staff keep the two lots of parents from meeting each other and ensure, by the narrowest of margins and high-speed moves of pupils from one classroom to another, that each set sees what they are expecting to see in the classroom and on the sports field. Matters are further complicated by the relations between the male and female teachers. Tassell and Joyce Harper, one of Miss Whitchurch's younger staff, become increasingly close (ending up together by the end of the play) while Miss Gossage, Miss Whitchurch's hearty deputy, becomes keen on Rupert Billings, Pond's blasé mathematics master, who is aghast at her interest in him.

At the end of Act 2 the deception finally falls apart: both sets of parents, the boys and girls, and the staff of both schools all run into each other. Miss Whitchurch faints into Miss Gossage's arms. In the last act the two head teachers make strenuous attempts to get the bureaucratic blunder resolved, while the parents become increasingly irate. At the end of the play a fleet of coaches arrives bearing the staff and pupils of another displaced school – this one co-educational. As the chaos mounts, Miss Whitchurch blows a piercing blast on a whistle, the action freezes and the play ends.