A disheveled, agitated young woman arrives unannounced at Hercule Poirot's flat one morning and blurts out that she thinks she may have committed a murder. Before she can explain, she takes one look at the famous detective, decides he is far too old to help her, and bolts. Poirot, both stung and intrigued, learns from his friend the crime novelist Ariadne Oliver that the visitor was Norma Restarick, one of three young women sharing a London flat. Norma's father, Andrew, has only recently returned from years abroad with a glamorous new wife, Mary, and Norma seems unable to say plainly what crime she fears she has done, or whether any crime occurred at all. As Poirot and Mrs Oliver move through the bedsits, art parties, and coffee bars of swinging-1960s London, they encounter Norma's poised flatmate Claudia, the elusive Frances, and David, the unkempt young man Norma loves and her family distrusts. Memory, identity, and the line between delusion and danger blur as Poirot tries to determine whether his troubled client is a witness, a target, or something else entirely. Set against a generation Poirot scarcely recognizes, the investigation turns on who Norma really is and what she has actually seen, building toward a resolution that reframes everything that came before.