Leo has brought a new toy rocket, and he and Caillou pretend to be astronauts traveling to the moon. Caillou's daddy reads in the newspaper that a new planetarium is opening, and they're holding a contest for the best space costume. Rosie gets upset when Caillou and Leo wouldn't let them play with her, so Mommy decides to make some space costumes. Leo gets dressed like an astronaut and Caillou is a space monster. At the planetarium the next day, Caillou and his friends play a game where they pretend to walk on the moon and explore space. Rosie and her mommy win the best space costume contest.