The Gaudy Dinner is an old student tradition at Oxford University, one that Harriet Vane is not entirely sure that she wants to attend as she prepares to go back to Shrewsbury College unsure of the reception she is going to receive when she gets there; after all, it wasn't too long ago that she was accused of murder, but to her surprise, a warm welcome awaits. Her old professors are happy to see her, and for a while she is overcome by sentiment and begins to remember what she enjoyed about student life. However, there is a cloud cast over her visit when she receives a poison pen letter that refers to her as a "dirty murderess".
A good while afterwards, the Dean of Shrewsbury College contacts Harriet and asks for her help. There has been a spate of vandalism and many people have received nasty anonymous letters. If this gets out then it will be very damaging to the reputation of the college, and he needs someone whom he can trust, and whom he knows to have the best interests of the college at heart. That is why he thought of Harriet. Although she really does not want to go back to the college for an extended stay, Harriet agrees. She comes up with a cover story and pretends that she is researching a paper about Sheridan Le Fanu, an Irish novelist famous for penning gothic horror stories.
Harriet learns that the vandalism has taken the form of offensive graffiti, and the destruction of several academic collections and research materials. She finds it difficult to narrow down the list of suspects and starts to become overwhelmed, the case and her musings upon it leading to some quite depressing introspection. She eventually enlists the help of Wimsey, about whom she has mixed feelings, but who turns out to be a very helpful fresh pair of eyes. The letters begin to take on a more sinister leaning when they are a clear attempt to drive one of the recipients of them, a young female student, to suicide. Harriet is attacked, and almost killed, by an unknown assailant. Things come to a rather threatening and violent head before the perpetrator is revealed.
She is Annie Wilson, a college housekeeper, whose husband was a disgraced professor at the University of York revealed to have participated in a large academic fraud. The fraud was exposed by an examiner and shame, and the loss of his career, drove Professor Wilson to take his own life. Annie is both distraught and angry, When she learns that the examiner who discovered the fraud has moved to Shrewsbury College she begins to plot his downfall. He has been the primary recipient of the poison pen letters, but she has also sent many to female students and faculty because she dislikes them for forging a path that she feels is above the station of women.She believes women should take on domestic roles, not academic ones.
Once Annie is arrested, things at Shrewsbury College take on a nice, more gentle, tone again. Wimsy asks Harriet to marry him. Her mind cleared by the introspection brought on by her investigation, she finally says yes.
