The Subtle Knife

While His Dark Materials is a contiguous trilogy, it is possible to read each book in a stand-alone fashion, although you would have to have nerves of steel to read one and not want to read the next one.

Opening up the 'Multiverse'

While The Golden Compass took place exclusively in a universe parallel to, and very similar to our own, The Subtle Knife begins in our universe, near our Oxford, and introduces the character Will Parry.

Will is a twelve-year-old boy, who lives with his mother and shoulders burdens too heavy for a boy his age to have to cope with. His mother, to the casual eye, would appear to be severely disturbed; paranoid, hiding from mysterious stalkers, her care devolves on her young son, who is aware that something is just 'not right' with his fragile mother. But in her case, she has good reason to be paranoid - sometimes they really are out to get you.

As it happens, Will's absent father has been lost in Lyra's universe, and the stalkers are trying to find information he left with his family in our universe. As they close in, Will accidently kills one defending his mother, and is forced to run. He runs directly to an unknown universe next to ours, where he meets Lyra, who left her own universe with no way to get back in the end of The Golden Compass.

The Subtle Knife

Will discovers how to use the Subtle Knife of the title, an arcane instrument that allows one to cut holes in the fabric between adjacent universes - the deeper the cut, the farther the universe you enter is from our own.

The universe between Lyra's and Will's is inhabited apparently exclusively by children. It seems that when children grow up, they can see and be seen by 'spectres', ghastly phantoms that destroy their souls and leave them zombies.

The young pair team up to discover what they can about Dust, eventually enlisting the bewildered help of an Our-Universe physicist named Mary Malone, who recognizes that Dust is the Dark Matter of modern physics.

Metaphysics

As Mary studies the Golden Compass and the interactions of Dust, her view of the natural world is overturned; Dust, or Dark Matter, acts in ways that is impossible to explain by modern methods, and the inescapable conclusion is that Dark Matter has intention, perhaps even sentience.

Angels, actual winged beings that purport to serve God, are meddling in man's affairs. Will and Lyra lose, and must recover the Golden Compass, and take it to Lyra's father Lord Azriel, whose intention is to use it to kill God and free mankind from the tyranny of religion.