When mysterious symbols appear as graffiti around Oxford, lexicographer Martha Thornhill recognises them from an alphabet created by a deceased writer whose papers she once worked on. But this is no publicity stunt, and tensions intensify when further graffiti is found near the body of a university lecturer.

Martha is not the only one to have doubts when the suspicious death is assumed by police as suicide. Called upon to lend their linguistic expertise to the case, Martha and her colleagues begin to unearth a tragedy that extends beyond the ancient walls of the university. As new messages portend more death, it's clear that a spirit of vengeance is stalking the city's streets, and may be reaching out towards them too.