When Charlie climbs into his tree house he also climbs into the tree's mind. Not only does he learn a thousand names for weather, but, in a shattering moment of insight, he sees the future of the earth.
Charlie begins a project to save his own garden, not knowing that he will soon be called to much greater things.
Meanwhile the tree, Ash, learns how to think like a human. He struggles to understand beauty, humour and imagination. At first he is full of wonder and admiration but when his friend, the rat tries to persuade him that humans are vermin, Ash begins to have doubts.
Eventually plants and animals sit in judgement on the human race and ask: might the planet be better off without them? Which way will Ash cast his vote?