When the traffic lights near school stop working a policeman comes to direct traffic. Caillou loves watching him and learns from Miss Martin about lots of different ways that police officers help people. Caillou decides he wants to help people too, and finds a whistle for directing traffic. Unfortunately his friends aren't very willing to be directed, but he does get to put Leo the bank robber in jail under the slide. He even finds a way to help the police officer… by bringing him a cool drink when he's done his work.