Time-Traveler-Rory

Summary: This is a sort of "Doctor Who" meets "Avatar" story, without the avatars. The Doctor, Amy, and Rory land on a human colony world where the colonists live in gigantic trees. A whole city in the treetops. But what starts out as a lark, ends with a murder, an ancient mystery, and  a local race of "not quite intelligent" spider-monkey-like creatures that might have a reason for not quite being intelligent... (This story is set between A Christmas Carol and Impossible Astronaut.) Up a Tree (p.2) Rory stepped down with relief onto another large branch that formed a wide platform, the stairs continued on beyond the other edge.