Back on Earth after his life-changing encounter with the people of the planet Arboretum, Timothy Martin learns of a developing crisis in the Galaxy. The Shadow is growing and threatens Earth's nearest populated planet with oppression and malice. Responding to this threat, the Custodians of Arboretum find themselves in mortal danger. Timothy accepts a plea to come to their aid. But will he be able to make a difference? Can an answer be found to combat the Shadow? Will the black sea of Timothy's inner world thwart his chance to save his friends?