While lost in the woods, Denny and Buzz Crocker come across a mysterious man named Flynn who unbeknownst to them is in fact a cab driver who died years ago as a result of a crash. Flynn helps lead them to someone who he says can help them get home, a very unusual man named Dr. Vink. On the way to Dr. Vink's cottage, Flynn inquires if they're any good at solving riddles, Denny and Buzz wondering why he would ask such a thing. Once Denny and Buzz arrive at Dr. Vink's cottage, they are also asked by him how they are at solving riddles. Dr. Vink tells the boys that if they want to get home that they must solve his riddles. His first riddle for the boys is an easier one "How far can you walk into the woods?" that Buzz is able to solve quickly with the answer being halfway. Then he presents a very tough riddle that he states nobody has been able to solve before: "What has no weight, can be seen by the naked eye, and if put in a barrel it will make the barrel lighter?". Denny and Buzz now have the opportunity to solve the it in order to call their parents, but Buzz is stumped. Vink informs them that they can get his help if the give him a specimen showing them somebody's hand he took that he put in a glass jar filled with water.
They leave the cottage and go to where Dr. Vink says a taxi cab will pick them up in the woods. A cab shows up mysteriously and the driver turns out to be Flynn and he says he thought he would be seeing the boys again saying that he thought they would be the ones to solve Dr. Vink's riddle. Then shows them that it was his hand Dr. Vink took for not being able to solve the riddle in exchange for a generous tip for taking Vink to his cottage saying he was in an accident that killed him. Denny and Buzz still have a little bit of time before the cab crashes into the tree that has been happening for the last 40 years since Flynn died in the first one all those years ago. And now they must try their hand at solving the riddle in order to save themselves from the fatal crash as well as free all of Dr. Vink's victims' spirits so they can finally be at rest.