In this third adventure, Agatha's mother gives the children two tickets to the cinema. Alfred is excited and really enjoys the show. Leaving the cinema the children meet a distinguished gentleman, none other than Thomas Alva Edison, inventor and businessman who invites them to visit his factory. However, this was more than a chance meeting, and the businessman has an ulterior motive for inviting them. When they get back home, Agatha discovers that her uncle has sent a mysterious package; a box that they're forbidden from opening. With instructions to deliver the parcel to a messenger, the mystery begins.