Lyra and Will, the ordinary children who were left in such extraordinary danger at the end of Book Two, find themselves in the final part of the story facing even greater perils. But there are old friends who come to their aid: not least Iorek Byrnison, the armoured bear, and the scientist Dr Mary Malone. There are new allies: the Gallivespians, hand-high dragonfly-riders with poison spurs, and the mulefa, strange wheeled creatures with the power to see Dust. And there are new dangers, including a journey to the worst world of all - a journey that exerts a terrible cost on both children.