Charles is an immigration officer with a difference: the migrants he deals with don’t come from other countries but from other universes. Known as shifters, they materialize from parallel timelines, bringing with them a mysterious drug called slip which breaks down the boundary between what is and what might have been, and offers the desperate and the dispossessed the tantalizing possibility of escape.

When a shifter-led gang commits murder in the name of the old Norse pantheon and then uses slip to escape justice, Charles and his girlfriend Jaz decide to cross to another universe themselves in a bid to confront the problem at source and prevent their own society from degenerating into tyranny.

But is that Charles' real motive? Or does he have other reasons which even he can't clearly see?