Harriet Vane returns with trepidation to her alma mater, Shrewsbury College, Oxford to attend the Gaudy dinner. Expecting hostility because of her notoriety (she had stood trial for murder in an earlier novel, Strong Poison), she is surprised to be welcomed warmly by most of the dons, and rediscovers her old love of academic life. Harriet's short stay is, however, marred by her discovery of a sheet of paper with an offensive drawing, and a poison pen message referring to her as a "dirty murderess".
Some time later the Dean of Shrewsbury writes to ask for her help. There has been an outbreak of vandalism and anonymous letters, and fearing for the college's reputation if this becomes public knowledge, the Dean wants someone to investigate confidentially. Harriet, herself a victim of poison-pen letters since her trial, reluctantly agrees, and returns to spend some months in residence, ostensibly to do research on Sheridan Le Fanu and to assist a don with her book. The timing of the first poison pen message during the gaudy, and the use of a Latin quotation from the Aeneid during one disturbance, focuses suspicion on the Senior Common Room dons, causing escalating tensions.
As Harriet wrestles with the case, trying to narrow down the list of suspects who might be responsible for poison-pen messages, obscene graffiti, wanton vandalism including the destruction of a set of scholarly proofs, and the crafting of vile effigies, she is forced to examine her ambivalent feelings about Wimsey, love and marriage, and her attraction to academia as an intellectual and emotional refuge. Wimsey eventually arrives in Oxford to help, and she gains a new perspective from those who know him, including his nephew, an undergraduate at the university.
The attacks build to a crisis. There is an attempt to drive a vulnerable student to suicide and a physical assault on Harriet that almost kills her. The perpetrator is finally unmasked as Annie Wilson, one of the college scouts, revealed to be the widow of a disgraced University of York academic. Her husband's academic fraud had been exposed by an examiner, destroying his career and driving him to suicide; his suicide note used the Latin quote eventually used by Wilson. The examiner later moved to Shrewsbury College, and the widow's campaign has been her revenge against the examiner in particular and more generally against intellectual women who move outside what she sees as their proper domestic sphere.
At the end of the book, with Wimsey admitting his own faults in his attempts at courtship and having come to terms with her own feelings, Harriet finally accepts Wimsey's proposal of marriage.
