The story revolves around a young boy named John who decides to play detective using a badge labeled "Detective No. 30" found in his jacket. He observes a suspicious man who parks a car, leaves the engine running, and enters a nearby house. John hides in a box to watch. To his surprise, a different man with grey whiskers and a cap comes out of the house, and the car leaves.
John shares this information with his father, who contacts the police. With John's help, the police identify the stolen car and connect it to the theft at Mr. Stone's house. The boys, who are aspiring detectives, reveal their game of tracking people with colored seals. The officers find a seal on the suspect's coat, confirming his involvement in the theft.
The story ends with the suspect admitting his guilt, the boys being praised for their cleverness, and John receiving a reward of a hundred dollars.
Author: L.M. Swenson
