The Aldens are helping one of Grandfather's friends break a world record by building the world's largest yo-yo! As they help with the project, strange things begin to happen. First someone follows them, and then the plans for the yo-yo turn up missing. Someone is trying to sabotage the yo-yo before it's launched.

It seems there are a few people in Greenfield who don't want this yo-yo completed. The Boxcar Children will piece the evidence together to get to the bottom of this mystery.