Soviet engineer Kuznecov is coming to Prague as an expert on work with the tunneling shield in construction of the metro. He is returning after more than thirty years. In May 1945 as a young soldier in the Red Army he was seriously injured in the liberation of Prague and while recovering experienced a great love with a young teacher called Vera.

A trespassing gang of boys encounter a ghost in the ruins of a castle. When they rescue the ghost who is caught in a trapset, they become allies in a fight to prevent ruins from being turned into mushroom farm by the city authorities.