Two friends would advertise for a third girl to help with the rent. And it is the "third girl," Norma Restarick, who is the subject of the baffling mystery that here engages Hercule Poirot. What is wrong with Norma? She walks in on Poirot at the breakfast table and announces that she "may have committed a murder" and then walks out again, leaving Poirot to battle his way to the truth.