Three tiny, ancient beings - Moss, Burnet and Cumulus, once revered as Guardians of the Wild World - wake from winter hibernation in their beloved ash tree home. When it is destroyed, they set off on an adventure to find more of their kind, a journey which takes them first into the deep countryside and then the heart of a city. Helped along the way by birds and animals, the trio search for a way to survive and thrive in a precious yet disappearing world ...