The 4½ friends have been assigned a real mission for the first time. Mr. Koslovski, president of the Gardeners' Association, assigns them to spy on Mr. Ritter, his opponent in the upcoming elections. Kalle and his group don't take their eyes off him. Since he goes to the gym every day, there's nothing left for them to do but exercise themselves – much to Friedel's dismay, who hates gym. What does the mysterious package that Mr. Ritter gives to two athletes contain? What is Mrs. Schulte-Stratmann, the Biology teacher, doing at the gym in an advanced state of pregnancy? And why does she specifically meet the man the 4½ friends are spying on? The young detectives will put on tracksuits to take part in the most suspicious endurance program and solve the mystery...