Mysterious events start happening in the city of Canterbury in England, baffling the local police force. The authorities are confronted with a series of murders having no apparent motive and little in the way of clues. The victims are an eclectic mix of visitors to Canterbury Cathedral, seemingly strangers with no obvious connection. The police work on two theories, a fancy dress party going horribly wrong or tit-for-tat reprisals instigated by an international spy ring. A code book, 'Codex Antonius', plays a vital part in the story, yet this adds to the confusion as it is written in Latin.