Taken at the Flood (1948)

A few weeks after marrying an attractive young widow, Gordon Cloade is tragically killed by a bomb blast in the London blitz. Overnight, the former Mrs. Underhay finds herself in sole possession of the Cloade family fortune. Shortly afterwards, Hercule Poirot receives a visit from the dead man's sister-in-law who claims she has been warned by 'spirits' that Mrs. Underhay's first husband is still alive. Poirot has his suspicions when he is asked to find a missing person guided only by the spirit world. Yet what mystifies Poirot most is the woman's true motive for approaching him...