After accidentally killing an intruder, Will escapes via a portal into another world and finds himself in the city of Cittàgazze where he meets Lyra and her dæmon Pantalaimon, who have arrived from her world via the opening in the sky created by her father, Lord Asriel (in Northern Lights). Twelve-year-old Will Parry lives in Oxford, where he cares for his mother who has mental health problems. The Subtle Knife of the book's title is a knife called Æsahættr, which can cut windows between worlds. By setting the alethiometer's hands to point to symbols around a dial a skilled practitioner can pose questions, which are answered by the movement round the dial of a further hand.

An important plot device is the alethiometer, a truth-telling symbol reader. In Lyra's world, humans' souls naturally exist outside of their bodies in the form of sentient ' dæmons' in animal form which accompany, aid, and comfort their humans.

The main setting is a universe dominated by the Magisterium, an international theocracy which actively suppresses heresy. Will Parry is introduced as a companion to Lyra, and together they explore new worlds in the search for Will's father. The novel continues the adventures of Lyra Belacqua (now known as Lyra Silvertongue) recounted in the first novel, Northern Lights, as she investigates the mysterious phenomenon of Dust. The Subtle Knife is a young-adult fantasy novel published in 1997 and the second book in Philip Pullman's His Dark Materials trilogy.

The subtle knife book