Harriet Vane returns with trepidation to Shrewsbury College, Oxford to attend the Gaudy dinner. Expecting hostility, she is surprised to be welcomed warmly by the Dons and rediscovers her old love of the academic life. The Dean requests her help with an outbreak of anonymous letters, vandalism, and threats, apparently from someone within the college. Harriet reluctantly agrees to help, and spends much of the next few months in residence, ostensibly to do research on Sheridan Le Fanu and to assist a Don with her book.
As she wrestles with the case Harriet is forced to examine her ambivalent feelings about love and marriage, along with her attraction to academia as an intellectual and emotional refuge. Harriet is also forced to re-examine her relationship with Wimsey in the light of what she has discovered about herself. Wimsey eventually arrives in Oxford to help her, and she gains a new perspective on him from those who know him, including his nephew, an undergraduate at the university.
The attacks build to a crisis, and the college community of students, Dons, and servants is almost torn apart by suspicion and fear. The perpetrator is finally unmasked by Wimsey.