While her famous employer is happily back home at Mallowan Hall, wrestling with her Belgian detective's dilemma on board the Orient Express, Phyllida Bright is finding her local renown as a sleuth has put her in high demand. A distraught Vera Rollingbroke suspects her husband of infidelity and has invited Phyllida to a dinner party to observe his behavior, particularly in regard to one Genevra Blastwick.

What she does observe at the party is that Genevra craves attention, in contrast to her shy sister Ethel. Genevra introduces a game called Two Truths and a Lie, and one of her questionable statements is that she once witnessed a murder. At this bold claim, the guests react with disbelief and pepper her with questions. Genevra remains cagey, withholding details, but insists this is not her lie.

The next morning Phyllida learns poor Ethel was purposely run down by a motorcar the previous night while inexplicably walking home alone from the party. She fears Genevra may have been the target, which means someone at the party is a killer—twice over. A chilling thought. With Genevra in potential danger—and Inspector Cork proceeding ponderously as usual—Phyllida takes it upon herself to unmask the killer.