Some time later the college Warden writes to ask for help. There has been an outbreak of anonymous letters, vandalism and threats, apparently from someone within the college, and a scandal is feared. Harriet, herself a victim of poison-pen letters ever since her trial, agrees reluctantly to help, and spends much of the next few months resident at the college, ostensibly to do research on Sheridan Le Fanu and assist a senior scholar with her book.
As she wrestles with the case, trying to narrow down the list of suspects and avert a major scandal, Harriet is forced to examine her ambivalent feelings about love and marriage, along with her attraction to academia as an intellectual (and emotional) refuge. Her personal dilemma becomes entangled with darkly hinted suspicions and prejudices raised by the crimes at the college, which appear to have been committed by a frustrated woman academic. She is forced to reexamine her relationship with Wimsey, who eventually arrives in Oxford to help her, in the light of what she has discovered about herself; and also gains a new perspective on him from those who know him, including his nephew, a current Oxford undergraduate.
The attacks build to a crisis, and the college community of students, dons and servants is almost torn apart by suspicion and fear. There is an attempt to drive a vulnerable individual to suicide, and a physical attack that almost kills Harriet. The perpetrator is finally unmasked by Wimsey as one of the servants, revealed to be the widow of a former academic. One of the dons previously exposed her husband's academic fraud, breaking his career and driving him to suicide, and the campaign has been her revenge against intellectual women who move outside their "proper" domestic sphere.
In an extraordinarily touching scene at the end of the book, Harriet Vane finally accepts Wimsey's proposal of marriage. (Their engagement, marriage and honeymoon (interrupted by yet another murder mystery) are depicted in Busman's Honeymoon.)
"Gaudy Night is a remarkable achievement. Harriet Vane and the grown-up nephew of Lord Peter help give variety, and the college scene justifies good intellectual talk. The motive is magnificently orated on by the culprit, a scene that in itself is a unique bit of work. And though the don-esses are sometimes hard to keep apart, the architecture is very good. Note a reference to C. P. Snow's The Search, and sound views on counterpoint versus harmony."
In 2005 an adaptation of the novel was released on CD by the BBC Radio Collection to finally complete the run of Wimsey adaptations begun with Whose Body? in 1973; the role of Harriet was played by Joanna David, and Wimsey, as ever, by Ian Carmichael.
The plot of Gaudy Night was adapted to become the two-part Out of the Past episode (#155 & #156) of the American television mystery series Diagnosis Murder starring Dick van Dyke as Dr. Mark Sloan. The episode first aired on 11 May 2000, with John Schneider as the villain.