富豪小说家哈兰·斯隆比在自己85岁生日第二天，被发现在自家庄园离奇自杀，遗留了亿万遗产。久负盛名的大侦探布兰科（丹尼尔·克雷格饰）突然被匿名人士雇佣调查此案真相。同时，哈兰的孙子兰森（克里斯·埃文斯饰）也正在秘密调查此案。当布兰科和哈兰·斯隆比家族的其他人对谈时， 他发现事情并没有想象中那么简单。
哈兰家族没有表面上那么和睦，每个人都有着不可告人的秘密，每个人都想得到遗产……究竟这起命案是自杀还是他杀？似乎每个人都有嫌疑。随着一位遗产继承人的意外亮相，真相谜底渐渐浮出水面……

A young man, broken down in the fog, witnesses a murder he is asked to conceal...A full-length novel adapted by Charles Osborne from Agatha Christie's acclaimed play. When a stranger runs his car into a ditch in dense fog in South Wales and makes his way to an isolated house, he discovers a woman standing over the dead body of her wheelchair-bound husband, gun in her hand. She admits to murder, and the unexpected guest offers to help her concoct a cover story. But is it possible that Laura Warwick did not commit the murder after all? If so, who is she shielding? The victim's retarded young half-brother or his dying matriarchal mother? Laura's lover? Perhaps the father of the little boy killed in an accident for which Warwick was responsible? The house seems full of possible suspects! THE UNEXPECTED GUEST is considered to be one of the finest of Christie's plays. Hailed as 'another Mousetrap' when it opened on 12 August 1958 in the West End, it ran for 604 performances over the succeeding 18 months and has been staged many times around the world over the last 40 years.