During the Soviet invasion of Czechoslovakia in 1968, Heim was in Prague employed as translator by UNESCO. When the tanks rolled into Prague, he was in the unique position of being able to translate between Czech and Russian, thereby facilitating communications between the Soviet soldiers and the Czechoslovaks on the streets. With his knowledge of German, he was also able to assist a West German television crew in navigating the occupied city and interviewing ordinary Czech citizens, and to warn potential victims that Soviet agents were looking for them.