On her way to meet her lover, Ruth Kettering is found strangled to death on the train. Missing is the valuable Heart of Fire rubies, which her father gave her. Poirot delves into the case, sorting through the stories of the passengers on the train, Ruth's lover, and her husband Derek Kettering. The eccentric and fastidious Hercule Poirot must untangle the web of lies, love, and changing affections to find Ruth's true murderer.