A woman suspected of murdering her doctor boyfriend has an identical twin sister. When both twins have an alibi for the night of the murder, a psychiatrist is called in to assist a detective in solving the case. Through a series of tests, he discovers which twin actually committed the crime and in the course of his investigation he falls in love with the normal twin. 弗兰克医生（Garry Owen 饰）被发现在自己的家中遭到谋杀，探长史蒂文森（托马斯·米切尔 Thomas Mitchell 饰）负责调查此案。弗兰克医生的邻居告诉史蒂文森，在他被杀的那晚，一位名叫露丝（奥利维娅·德哈维兰 Olivia de Havilland 饰）的女子曾经拜访过他家，于是史蒂文森找到了露丝，哪知道后者有坚不可摧的不在场证明，案件的调查陷入了窘境之中。 之后，史蒂文森得知露丝有一个双胞胎妹妹名叫特里（奥利维娅·德哈维兰 Olivia de Havilland 饰），心理学专家斯科特（刘·艾尔斯 Lew Ayres 饰）调查后发现，特里疑似患有非常严重的精神疾病，甚至连和她血脉相连的露丝都也许会有生命危险。
