11 years old Matt inherits an oak seed and a box full of soil from a distant Scottish relative.
Matt is a modern boy and does not know what to do with the 2 gifts, but he is persuaded by his friend Chloe, the daughter of a forest guard and an environmental supporter, to plant the seed in his garden, leaving a long-lasting mark for his descendants, as the tree grows.

Yet nothing goes as planned. The seed is really an egg of the dying Green Man, the undisputed lord of the natural world, seeking a Green Knight who can help him to be regain its full strength.
Within a summer, a beautiful oak tree sprouts from the seed and generates Gryn, a young dryad, with the ability to talk to plants. Gryn binds telepathically to Matt and urges him to save the natural world in order to preserve life on Earth.

Thanks to Gryn, Matt and Chloe discover that the oak tree is also a hidden passage that allows them to travel almost instantaneously from one place to another on Earth and even on other planets wherever plant life is present.
The journey is granted by the Leys, the power lines running through the living tissue of the whole universe.