From The Perry Mason TV Show Book (Revised)
This is a most unusual Mason episode. Della and Paul do not appear. There is no defendant, and therefore no trial. Although Perry is in France on other “business,” he nevertheless manages to get embroiled in an adventure involving Nazis, hidden treasures, and not one but two murders.

Perry is in Paris when he gets a call from prominent American Frederic Ralston II. Ralston wants Perry to go to Germany and check up on his son, Frederic III. Freddy, who is a lieutenant in the Army Corps of Engineers, has been courting a German singer, Greta Koning, and plans on marrying her. But when Perry arrives in the lakeside village called Mitternacht, he is quickly involved in a murder. Freddy is found standing over the dead body of an army captain who was investigating a report of Nazi treasure hidden at the bottom of the lake. But Perry soon learns that the only thing at the bottom of the lake is another body.

Werner (Hogan’s Heroes) Klemperer, who’s made a good living playing unscrupulous types such as spymasters and Nazis, appears as Inspector Hurt.