The return of Harriet Vane, after two novels in which she makes one cameo mention. Indeed, the through-going return -- there are one or two scenes from Lord Peter Wimsey's point of view, but overwhelmingly this is Harriet's novel.
Like The Nine Tailors, this starts with setting the scene. Harriet returns to the (fictitious) college Shrewsbury, at Oxford, from which she graduated. She meets old fellow students -- sometimes happily, sometimes not -- and the teachers, and is plagued by a poison-pen writer, and talks with people about detecting and crime in fact and fiction.
Which explains, even though she tells no one of the poison pen bits, why they call on her for help when they realize that a poison-pen writer is plaguing the college, and doing graffiti, and otherwise being a nuisance. Her information enables them to clear the new students and realize that most likely the culprit is one of the dons.
She stays to help investigate. The novel involves the plot holes in her latest work, a set of chessmen, May morning at the Magdalen Tower, punting on the river, a passage from Virgil, a dress no one's seen, two children of a scout who stay with a family nearby, Lord Peter's nephew being recognizable by voice, many quotations from innumerable sources, and much more.