The 4½ friends have been assigned a real mission for the first time. Mr. Koslovsky, the president of the Perivolari Association, assigns them to spy on Mr. Riter, his opponent in the upcoming elections. Kalle and his gang do not let him out of their sight. Since he goes to the gym every day, there is nothing left for them to do but to exercise themselves - much to the dismay of Friedel, who hates sports.

What does the mysterious package that Mr. Riter gives to two athletes contain? What is Mrs. Sulte-Stratman, the Biology teacher, doing in an advanced state of pregnancy at the gym? And why does she specifically meet the man that the 4½ friends are spying on? The little detectives will wear uniforms to participate in the most suspicious endurance program and solve the mystery...