One day, Phelim O'Green, a boy living with his sister Prudence shortly after the First World War, wakes up to discover his house filled with strange, filthy creatures called glashans, led by a creature who lives behind his stove called the Domovoy. They are hiding from a monstrous black dog. They force Phelim out of the house, referring to him as 'Jack'.
He meets a witch with no shadow or reflection called Alexia and a madman lives in trees and has inexplicably been alive since the Battle of Waterloo called Sweeney. They explain to him that a continent-sized worm called the Stoor Worm has been awoken by the sound of artillery from the war. As it wakes, it's eggs hatch, releasing various mythological monsters who are now rampaging across Europe. Phelim is 'Jack O'Green', a figure destined to slay the worm and save the world.
- Aborted Arc: On the way to Hy Brasil, Uncle Murdo tells Phelim that his companions are working for the Stoor Worm and the Obby Oss is a hatchling. This is never mentioned again; this is odd because Murdo had little reason to lie, yet Sweeney, Alexia and the Obby Oss never show any signs of being aligned with the Stoor Worm.
- Abusive Parents: Prudence. Actually an abusive older sister, but she still raised Phelim, destroying his self esteem by constantly insulting him, and lied to him about his father; she claimed he had died when she actually had him committed to a lunatic asylum for being a 'dreamer'..
- Apocalypse How: Type 0. British civilisation is shown to have mostly collapsed, with people reverting to Pagan practices out of fear, due to being attacked by hundreds, perhaps thousands of supernatural monsters. Because of the size of the Stoor Worm, it can be reasonably assumed that the same is occurring in mainland Europe.
- Anticlimax Boss: The whole book is a build-up to the slaying of the Stoor Worm, and a large amount of tension is built around how hard it would be the fight it. Phelim kills it by kicking a mouse off a cliff.
- Casts No Shadow: Alexia, because the devil mistook it for her and took it. She regains it after being resurrected.
- Came Back Strong: Alexia/Aisling, subtly. After he resurrection, her white hair turns red, her shadow and reflection are back, and her features are slightly different, implying that the damage done to her by studying dark magic has been repaired.
- Our Dragons Are Different: The Stoor Worm is referred to as a dragon several times, and actually shares more traits with a dragon - such as having teeth and a snout, and laying eggs - than with a worm.
- Our Mermaids Are Different: The merrows - they're monsters who steal souls and imprison them in crab cages.
- The Unmasqued World: Before the events of the book began, everyone had long since dismissed magic and mythology as mere superstition. With the hatchlings now rampaging around Britain and presumably, Europe, this is no longer the case.
- Wrong Genre Savvy: Uncle Murdo thinks that he's in the kind of work in which fairies are benevolent, wish-granting beings. He's not.