Awesome story featuring that one guy from the Christmas Carol episode

The Doctor is investigating a distress signal and lands on an enormous tree. He then has to figure out where he is and what the danger is, and it gets crazy. At the base of the tree he finds a pool of boiling sap, and at the top he finds insects and empty space, the tree is just free floating and is huge enough to have its own atmosphere.

The villagers traditions are very interesting to see play out (they protect the young and mothers based on these from the threat they don't understand) and the ending twist is pretty funny as well.