Marc, Ueli and Reto love airplanes. Whenever an interesting aircraft is on display, they meet at the airfield near Lucerne. One day, they notice a strange man secretly taking pictures with a camera. Ueli has a bad feeling about this and decides to follow the stranger. He overhears him talking to another man and finds out that the three men want to tamper with one of the aircraft, the Arbalète research plane, at night. This makes not only the three friends, but also Ladina, Reto's twin sister, suspicious, and they decide to help the police with their own investigation. They even go so far as to break into the spies' house. But they are not alone...
