The lawyer Emil Fencl (Otomar Krejca) has retired and returns from his countryside place of work to Prague, to his own villa inhabited by his nephews - Karel with his wife Jitka and the single Michal. On the train, he meets a pleasant criminologist named Kudrna (Karel Höger). Michal (Jan Tríska), a not too successful motorcycle racer, picks up his uncle at the railway station. From their conversation it becomes obvious that he is very jealous of Karel, a successful scientist who is soon to leave for a congress in the United States. Upon their arrival at the villa, there is a tragedy. Pressing the doorbell causes a gas explosion and a subsequent fire. Karel is found dead in his study. It seems that he himself took his life. The investigator, Captain Kudrna, finds out that Karel had just discovered the new element luridium and had no reason to commit suicide. Despite this, all attempts to prove foul play fail.