Gabriel Jordan lives a lonely life on the streets, taking the rare job that usually means spending hours in the cold, waiting for a chance to take a photograph of a cheating spouse. That’s how he meets Dr. Aleksei Tennant—who he sees suspiciously jumping from a window of a woman’s apartment. Tennant introduces Gabriel to the hidden worlds, magical worlds connected to this one by portals. Gabriel is eager to learn the runes for opening them, Tennant is eager to teach, and a fragile friendship develops amidst a series of murders. These aren’t just any murders. The killer has a grudge against the mixture of other races: he doesn't kill humans, only those of the hidden worlds who are producing children of tainted blood. When the killer attacks them directly, Gabriel and Tennant have to team up to find the murderer, even though they’re putting themselves, their love, and any chance at a future together, in danger.