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 to be a 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.