When young Tamwyn, determined to solve the mystery of the vanishing stars, seeks a vision in the mountains, he is confronted by a vision he never expected: From the place of the stars, dark shadows, omens of evil, are pouring out toward Avalon, posing a threat to the Great Tree's very existence. Tamwyn and his loyal troupe must split up: Elli to rush to the Lady of the Lake, Scree to heal with the Eaglefolks, and Tamwyn to journey to the Heartwood of Avalon itself to discover the portal to the highest branches and confront the very powers of Darkness.