Four Junior Detectives (1992)

The four young snoopers Tim, Karl, Kloesschen and Gaby by chance discover a hidden cave near an old castle. In it there's a board that seems to describe the way to a legendary sword. They ask their history teacher Schindler about it. He forbids them further actions, but secretly continues investigations with help of two ruthless criminals. But our young heros don't give up on an adventure so fast...