Following an otherwise enjoyable literary luncheon, the celebrated crime novelist Ariadne Oliver is accosted by a particularly formidable woman who engages her in conversation about one of her many goddaughters, whom Ariadne hasn't seen since she was a child. She lands a bombshell with the question "What I want to know is this: Did her mother kill her father, or was it the father who killed the mother?"Affronted and bewildered by the assertiveness of this pushy busybody, Ariadne is nevertheless intrigued. There's only one thing for it. She must contact her old friend Hercule Poirot.

Together they set about unravelling the mystery of a tragedy from the distant past—provoked in equal measure by hate and love.