Justus, Peter, and Bob are best friends and detectives. Together, the three ??? have already solved many tricky cases. A wealthy family's villa has been broken into. 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 these are typical signs used by thieves to indicate good loot or other things to their accomplices. Now, they need to prevent the next burglary and catch the culprits!.