S02E06 - The Adventure of the Cheap Flat

Hastings is at a friend's house with several other people when the talk turns to flats and houses. A young couple by the name of Robinson are there and she tells the party how they have managed to obtain a flat in Knightsbridge at an incredibly cheap rental that no one seemed able to obtain, despite its stunningly attractive price.
The next day, when Poirot is told of this singularly strange event, the Belgian Detective is immediately interested and starts to investigate. When they go to the buildings where the flat is, the porter tells them that the Robinsons have been there for six months, despite the fact that Mrs Robinson told 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 in order that he can enter at will. The next day, Poirot tells Hastings that Japp informed him 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 somewhat close match to that of Mrs Robinson.