Lily, Oliver Owl and Pip Squirrel join a ranger to investigate a mysterious blue patch that has appeared in a recent photograph of Green Valley Nature Reserve. By comparing the photograph with an old map and observing water, animal tracks and a beaver dam, they discover that a narrow stream has transformed into a thriving new wetland. A fascinating nature adventure that teaches observation, cause and effect, map reading, environmental change, and the importance of protecting newly formed wildlife habitats.