In this exclusive authorized edition from the Queen of Mystery, the Third Girl sharing a London flat with two others announces to Hercule Poirot that she's a murderer and then disappears. The masterful investigator must figure out whether the missing girl is a criminal, a victim, or merely insane. Three young women share a London flat. The first is a coolly efficient secretary. The second is an artist. The third interrupts Hercule Poirot's breakfast confessing that she is a murderer--and then promptly disappears. Slowly, Poirot learns of the rumors surrounding the mysterious third girl, her family, and her disappearance. Yet hard evidence is needed before the great detective can pronounce her guilty, innocent, or insane...