As Ann Dorland tries to stay away from journalists, Inspector Parker, Lord Wimsey and Bunter search her flat. They find a lot of detective novels and books on medicine and chemistry, which make Ann an even more likely suspect. Bunter takes photographs of the furniture and the paintings. Wimsey is particularly interested in the only portrait of a man, but the cubist style makes it impossible for him to recognise the face.
When two policemen stop by Sheila and George Fentiman, George panics. The already highly-strung young man rushes to the backyard, crushes a little bottle under his feet and runs away. His landlady, Mrs Munn, notices the shards from the bottle and decides to hide them from the policemen searching the flat.
Parker and Lord Peter interview the general's nurse, who claims that Ann couldn't have put the digitalis in his brandy. The poison in a liquid form would have killed him much sooner anyway.
When George doesn't return home that night, a terrified Sheila calls Lord Peter. Wimsey tries to comfort her and attracts the attention of Mr and Mrs Munn, who think there's something immoral going on. As Lord Peter senses that Mrs Munn is keeping something to herself, he calls for Bunter to bring some whiskey. The alcohol makes the Munns talk about the events of earlier that day. Mrs Munn says she's willing to hand over the crushed bottle in exchange for some money. Lord Peter refuses to pay, as he believes George is innocence. The young man, still suffering from the effects of the war, was just behaving impulsively. Sheila admits that the bottle belonged to her: it contain pills for her heart condition. Unfortunately, digitalis is one of the medicine's ingredients. So now George not only has a motive but also the means for the murder.
Bunter develops the photographs he made and turns them into slides which he projects against the wall. The portrait of the mysterious man is accidentally projected against the curtains, warping the picture, but actually restoring the man's real face.
At breakfast Lord Peter shows the adapted portrait to Parker: it's Dr Penberthy. Parker tells Lord Peter that Ann Dorland has returned to London; she's staying with Marjorie Phelps. Lord Peter goes to see her at Marjorie's houseboat and gives her his interpretation of the facts. Dr Penberthy and Ann had an affair and Penberthy wanted Ann's inheritance to start his new clinic. But shortly after the General's death their relationship ended. (Lord Peter remembered that Marjorie had said Ann was recovering from a bad break-up.) Ann admits that there was an affair and that Dr Penberthy had suddenly called it off, apparently because he felt she was buying him. Lord Peter explains that Penberthy must have panicked when an autopsy was ordered on the general's body. The discovery of the poison, combined with the news of a relationship between Ann and the doctor would have made the latter a suspect. Ann realises that she can't prove her innocence in the case.
George Fentiman gets picked up by the police and the confused man confesses to the murder of the general. Lord Peter and Dr Penberthy are called to the police station, but it's very clear that the young man doesn't know what he is saying. Lord Peter and Dr Penberthy return to the Bellona Club, where Lord Peter has a private conversation with the doctor in the library. He explains that he's figured out the murder, that he knows Dr Penberthy is guilty but that it's difficult to prove Ann Dorland's innocence. He asks the doctor to write a confession. Then Colonel Marchbanks walks in, leaving behind his gun.
The story ends where it began, at the bar in the Bellona Club. A shot is heard in the library and someone soon complains that there has been another unpleasantness ...