It is Midsummer, 1631. While Alexander Seaton and his fellow masters enjoy the holiday with their students, Robert Sim, librarian of Marischal College, is murdered in a dark alleyway in town. While the university and town authorities investigate the murder, Seaton is asked by the college principal to look into Sim's private life. In the course of an investigation in which his personal feelings threaten to cloud his judgement and endanger his young marriage to Sarah, he discovers a side to the librarian he could never have guessed at.