When the gang goes on a quest to find out what happened to the original Mystery Incorporated group on one of their last cases of the 75-year-old disappearance of the Darrow family and their mansion, they find themselves trapped in the underground Darrow Mansion, which has many booby-traps. What they do not know is that they are not alone inside the Darrow Mansion, and that someone is trying to kill them, in order to protect something which the original Mystery Incorporated was looking for. In the end, the gang solves the mystery and they collect a strange puzzle piece, which they believe might be cursed and that is essentially one of the key elements of that mystery, vowing to find out what exactly is the story behind it.