The Adventure of the Cheap Flat — Agatha Christie

Hastings is at a friend's house with several other people when the talk turns to flats and houses. Mrs Robinson tells the party how she and her husband have managed to obtain a flat in Knightsbridge for a very attractive price. Poirot is interested in this strange event and starts to investigate. The porter at the flat tells them that the Robinsons have been there for six months, despite Mrs Robinson telling Hastings they had only just obtained the lease. Poirot rents another flat in the building and, by use of the coal lift, manages to gain entry to the Robinson's flat and fix the locks so he can enter at will. Japp tells Poirot that important American naval plans were stolen from that country by an Italian called by Luigi Valdarno who managed to pass them onto an international spy, Elsa Hardt, before being killed in New York. Hardt's description is a close match to Mrs Robinson.