Jonathan's life changes when he leaves the safety of his London home for the Darkside, where vampires and werewolves are real, and far from the most deadly of the creatures who live there.

Jonathan knew his life wasn't normal, he just didn't know why. Other kids had mums and dads, and perhaps brothers and sisters, and they talked and laughed and did things together. Other kids had friends to hang out with, go to the movies, share secrets and stories. Jonathan didn't have any of that. His father was cold and distant, hardly ever speaking to his son. He got angry when Jonathan asked about his mother, and every so often he'd go into a coma, and had to stay in a mental hospital until he woke up, with no way to predict when that would happen. Once it was six months before he woke up. And when he was awake, he spent most of his time in his study, with the door locked. Jonathan had never even glimpsed what was inside. 

All that changed when one of Alain's spells come on so fast, he didn't have time to lock the door before he passed out. That night, Jonathan saw the open door about the same time he realized there was a stranger in the house. He barricaded himself behind the study's locked door, a door that had long, deep scratches on it, left by the intruder. The next day, Jonathan explored the study, finding old books on history or political science, diaries, journals, poetry—all of them dusty and deadly boring. Some of them had bookmarks or marked passages, all mentioning something about darkness or darkening. Alain had also scribbled notes to himself on scraps of paper, and since he'd dated them, Jonathan was able to put them into order. The most recent one was dated two days earlier, the day before his dad's last spell. It said, "A crossing?  Surely I must be close now." Below that was a book title, The Darkest Descent, with a page number and a library call number.

The book had been written by a famous Victorian explorer, and the page Jonathan was looking for was about the man's search for the darkness in the heart of London, talking to the most shameful and lowest creatures in the city. He finally found someone who would show him the gateway to the darkness, under the Blackfriars Bridge. That had to be the crossing his father had been looking for, and Jonathan knew he had to find it.

But getting to the Darkside wasn't easy, and Jonathan didn't know what to do when he got there. He had only one name, Carnegie, someone supposed to have been his father's friend. Unfortunately, Jonathan meets him on the night of the full moon, when he's not a man, but a beast.