Justus, Peter, and Bob are best friends and detectives! Together, the three ??? have solved many tricky cases. A break-in has occurred at the villa of a wealthy family. During the investigation of the crime scene, the three ??? kids notice a strange chalk mark. They had just discovered a similar mark on the fence of the junkyard shortly before. The three detectives find out that it is a typical sign used by thieves to indicate good loot or other things to their accomplices. Now it's time to prevent the next break-in and catch the culprits!.