Ziggy, Rashawn, Jerome, and Rico -- the Black Dinosaurs -- are thrilled to discover that their hometown was a stop on the Underground Railroad. Even more exciting, their new friend Mr. Greene has given them an old map that shows a secret passage, dating back to the days of the Railroad, right under their own school! How can the Black Dinosaurs resist making plans to check it out? When a trapdoor slams behind them, locking them in the tunnel, there's only one thing they can do -- plunge deeper and deeper into the darkness. Where will the tunnel lead them? And will the old, crumbling walls hold until they find their way to the end?