On one of their annual jaunts, young Kazran Sardick and the Doctor find themselves in trouble when the TARDIS is tangled in the branches of a very strange, very large tree.

They emerge into a habitat where myriad species fight for survival: an ecosystem of deadly flora and fauna, along with a tribe of primitive humans.

This is a mystery which can only be solved by climbing. But what will they find at the top of the tree?