The second volume of His Dark Materials introduces Will Parry, a boy from our world who discovers a window into the abandoned city of Cittàgazze, a place between worlds where soul-devouring Spectres drift through the streets. Will's acquisition of the subtle knife - a blade capable of cutting windows between worlds - makes him central to the conflict Lyra has stumbled into, and their alliance deepens the novel's themes around consciousness, free will, and the nature of authority. The Subtle Knife expands the multiverse considerably, introducing multiple parallel worlds and the angels who move through them, while grounding its cosmic scope in the relationship between two children navigating forces far beyond their understanding.