Tree houses are supposed to be magical hideouts. They are high up, shrouded by forest, and far enough out of their small town that no one can stumble across it.

At least that was Riley and Ace's belief. That is until Sammy and his followers destroy it, sending Ace and Riley into the woods. While hiding, Riley and Ace discover a forgotten train track and the tunnel that runs underneath. Riley instantly rejects the idea of moving their hideout into the tunnel.
It's cold. It's stinks. It gives Riley a horrible feeling.

Her suspicions are confirmed when Ace disappears from the tunnel and Riley turns towards a strange local kid who had a similar experience. Together they work to figure out who or what is taking the children and discover how to save her best friend.