Marple is asked by her lawyer, Christian Gilbranson, to visit his step-mother, Carrie Louise Serrocold, an old friend of Miss Marple's, at her country estate. Carrie's husband, Lewis, confides to Miss Marple that he suspects someone is slowly poisoning his wife, and when Gilbranson visits the mansion, he is mysteriously murdered.

A mathematician and author, Luke Williams, is travelling up to London on a train when he meets a old lady, Lavinia Fullerton, who is also going to London, to Scotland Yard. Lavinia tells Luke that in her small village several people have died. The local police are certain that it was all accidental and are taking no action but Lavinia isn't convinced. In London Luke watches, horrified, as Lavinia is run over in a hit and run and he becomes convinced that she was telling the truth. He travels down to the village and with the aid of a local girl, who is also convinced that the deaths were murder, sets out to solve the mystery.

1917年，俄国革命爆发，沙皇和他的家族被捕处死。但越来越多谣传说沙皇最小的女儿——最高贵的公主殿下安娜斯塔西亚（英格丽·褒曼 Ingrid Bergman饰）依旧侥幸存活着。安娜公主将是王朝在英国银行巨额存款一千万美元的唯一继承人。1928年，俄国将军波宁（尤·伯连纳 Yul Brynner饰）受流亡在北欧的俄国皇太后（海伦·海丝 Helen Hayes饰）之托，开始寻找公主的下落。几经波折仍寻公主无果，忽然一个精神错乱的女人（英格丽·褒曼 Ingrid Bergman饰）被波宁在一家精神病院发现。她有着与公主安娜惊人相似的容貌。经过波宁亲自监督的毫无人情味的训练下，她完成了从灰姑娘到贵夫人的转变。然而流亡丹麦的皇太后不为所动，任凭她如何努力都不相信她是真正的公主。 　　由安纳托尔·李维克执导，英格丽·褒曼和尤·伯连纳联袂主演的经典电影《真假公主》，荣获1957年奥斯卡金像奖最佳女主角，使女主角英格丽·褒曼成功在好莱坞复出。