《第三个女郎》讲述了三个年轻女郎在伦敦城内合租一套公寓。第一个是冷静而能干的私人秘书；第二个在艺术圈里混事。第三个女郎在早餐时出现了，当赫尔克里•波洛享用着奶油蛋卷和巧克力时，她冲进来告诉他，自己是个杀人凶手——转而又消失不见了。 波洛后来一步步探悉围绕这神秘的第三个女郎的种种传闻——她的家庭内幕，以及她的失踪，等等。然而，在伟大的侦探宣布这第三个女郎是否有罪，抑或是否精神错乱之前，他需要提供强有力的证据。 A perplexed girl thinks she might have killed someone...Three single girls shared the same London flat. The first worked as a secretary; the second was an artist; the third who came to Poirot for help, disappeared convinced she was a murderer. Now there were rumours of revolvers, flick-knives and blood stains. But, without hard evidence, it would take all Poirot's tenacity to establish whether the third girl was guilty innocent or insane!