The Case of the Diligent Detectives begins with Bobby running the meeting. It is his turn to be President. He wants the young detectives to become more diligent in their investigations. He overrules various suggestions for investigation and chooses investigating the brown blotches that are appearing on the grass. He suspects that alien spacecraft are hovering over the city at night and shining powerful laser beams at the ground, thereby causing the death of the grass in various places. He decides that he needs to sit in a pup tent in the park and take photos of any flying saucers that come by. The others agree to investigate but decide to trick him by building a small flying saucer that they can float over his tent. Dick builds the device and paints his hockey mask green. He gets dressed in green pajamas. After dark, the three conspirators arrive in front of Bobby's tent and start making alien noises. The trick backfires. Bobby is not amused. The next night, to make up for their insubordination, the three other detectives camp out and investigate diligently. What happens has to be read to be believed. The story provides an opportunity for the reader to learn Morse code, a little French and the unexplained mysteries of aliens who come in the night.