14 year old Sven is a technical genius, who has rebuilt an old caravan into a radio communication center. Via radio he is in contact with his father, who works at a big ship at the other side of the globe. One day the radio communication is suddenly interrupted by something which sounds like a Latin mass. In school Sven attends a lesson about radio-astronomy. The teacher explains that astronomers can receive radio waves that are thousand years old. Sven understands that the Latin mass he heard was transmitted to him from another age. He gets curious and starts making experiments by connecting his radio kit to bigger aerials. When he sits in the caravan together with his friends Bo and Petra, they not only hear the Latin mass on his radio, but the whole caravan is turned into a time-machine that brings them to the Middle Ages. They get in trouble with some warriors, who capture Petra, while Sven and Bo escape with the help of their time-machine. The two boys have to go back to the Middle Ages once more to rescue Petra, but this time Sven's little sister Hanna has sneaked into the caravan as well. In the Middle Ages some holy men finds Hanna and the caravan, which they think are sent from Heaven. After a sacred ceremony they let the children go, but the event stays in their memory as the Miracle in Valby.