Justus, Peter and Bob are best friends and detectives! Together, the three ??? have already solved many tricky cases.
The villa of a rich family has been broken into. While investigating the crime scene, the three ??? kids notice a strange chalk mark. They had just discovered one like it on the fence of the scrap yard. The three detectives find out that it is a typical crook's mark, with which thieves point out good loot or other things to their cronies. Now it's up to them to prevent the next burglary and catch the culprits!