A boy, a girl, a tramp and a piano with the most amazing effect on people, find each other in a park in a heatwave.

Maria comes from abroad to visit an English family. On the way to the Cinema Club she gets lost and misses the film. Next day the children find the cinema unlocked. They take her over it and try out the apparatus. Then they surprise a thief and give chase, finally trapping him with the spotlight so that the police can catch him.