A tree house is the office of Private Investor Scooter Kane and his reluctant partner and sister, Mary. From the lofty office, the imaginative partners pay close attention to what they believe are suspicious activities of the neighbors and have reported some of their findings to their father, the City Prosecutor for Elm City, Iowa. But so far none of the partner's observations have proved to be criminal, illegal, or even morally improper.
So it is not surprising when one evening, eleven-year-old Scooter and twelve-year-old Mary discover a wooden box in the street while skateboarding near their home. They know it is none of their business, but Scooter talks Mary into opening the box to discover it is full of money.
This is a whole new experience for these two detectives who have been lucky to solve mysteries of missing cats or bicycles. They realize adult advice is needed so they run the half-block home to get their dad who is working in the garage. Scooter begs him to check out their discovery; he tugs at his sleeve stressing the urgency. But, as parents often do, Dad moves at his own slow pace and by the time the kids get him to the site, the box is gone.
Dad is a non-believer in the story Scooter and Mary tell him. He reminds Scooter of another false report involving a Chicago bank robber that caused him some embarrassment at the court house.
The kids then realize they are on their own to solve a case that have the professionals baffled.