Celery's brother, Zucchini, and his pet, Moonbeam, decide to visit the Propulsions, but get lost along the way. The kids have to figure out where on Earth he is. As it turns out, Uncle Zucchini never made it to Earth, but has landed on a place that sort of resembles Earth - Titan, Saturn's largest moon! Titan has Earth-like features such as oceans, mountains, and an atmosphere.