The Ryves brothers, Nick and Alan, are hanging in their kitchen when they are attacked by a bunch of ravens that crash through the window. As they fight off the ravens (one of which turns into a human upon being killed), we learn a couple of things about these guys:
Needless to say, this is not the first time they've ever faced supernatural attacks, and it won't be the last.
While Nick and Alan are cleaning up the mess in their kitchen, three more characters are introduced. First there's Mum, apparently their mother, who is very curt, especially with Nick. Next come Mae and Jamie, two more teens, who have come to the Ryves brothers for help.
It turns out that Jamie, Mae's younger brother, has been marked by a demon, which means he's in deep doo-doo. As Nick and Alan explain, the marks mean that someone has to die. Mae begs the brothers to help, and Alan, who has a crush on Mae, is receptive. Nick? Not so much. But then Alan gets a demon's mark too, and before they know it, they're all in this mess together.
The four teens travel with the mysterious Mum (a.k.a. Olivia) to London where they set up a base camp so they can try to capture and kill two magicians, because that turns out to be the only way to remove Alan and Jamie's marks and save their lives.
As they plot and plan, we learn that we—along with a few of the characters—have been misled on several fronts. First off, Nick and Alan aren't technically brothers. Mum/Olivia is Nick's mom, but not Alan's, and it turns out they have different dads, too. Important side note: Nick's pop just happens to be the guy that's been hunting them down all this time. Yippee.
Second, we believe Nick's dad is hunting them because Nick's mom (Mum/Olivia) stole a valuable charm from him when she left him years ago. In truth though, Nick's dad wants Nick… but not because he misses him. He wants Nick because Nick is a kind of magical science experiment—he's not human at all. Turns out his dad, a powerful magician, arranged for his son to be possessed by a demon while he was still in utero. Nice, huh?
Of course hearing all of this is a bit trying for Nick (especially since at the time it's revealed he's trapped in an imprisonment circle and Mae, Jamie, Alan, and Mum/Olivia are all being held captive as well). But it also explains a lot of things he's wondered about for a long time.
In the end, Alan, who has always believed in the humanity of his demon brother, surprises everyone by setting him loose on the world, and remarkably, Nick doesn't destroy it. Instead he chooses to remain in his human body and try to learn to live (and let live) like humans do. Most of the time.
Though Mum/Olivia dies in the final confrontation, the rest of the main characters escape the bad guys—at least for now. There are a few issues that remain unresolved, like what will become of Nick as he tries to reconcile his demon core with his human body, and also what may lie in store for Jamie, who as it turns out, is a magician. But hey, those are matters for the sequels.