The Mystery of the Cheap Flat — Agatha Christie

When Poirot investigates the rental of an unusually cheap apartment, he finds that two spies attempt to rent their apartment to someone with the same name and similar appearance to avoid retribution from the mafia. Captain Hastings finds out from Gerald Parker about an unusually cheap flat rented to Mr and Mrs Robinson in Montagu Mansions near Knightbridge. Mrs Robinson fears that the low price may be an indication that the house is haunted.