It's a big week at Cogwheel Stadium—the baseball team the Clayton Cogs are playing for the pennant! The Aldens are helping out at the ballpark, and they're glad to be part of the excitement. But the other team's batters seem to know the pitches too well, and it's clear someone in the stadium is stealing the Cogs' pitcher's signals. Now the Boxcar children must find out who is spying for the other team!