The Adventure of the Cheap Flat

Hastings and several others are spending the evening with his friend Gerald Parker when the talk turns to house-hunting in London. Mrs Robinson tells the party how she and her husband have just rented a flat in Montagu Mansions, Knightsbridge for a very cheap price.

The next day, Hastings recounts the story to Poirot, who is intrigued and decides to investigate. Poirot and Hastings visit Montagu Mansions, where the porter tells them that the Robinsons have been there for six months, despite Mrs Robinson previously saying they had only just arrived. Poirot rents another flat in the building and, by use of the coal lift, gains entry to the Robinsons' flat and fixes the lock so that he can enter at will.

Via Inspector Japp, Poirot learns that important Naval plans were stolen from an American Government department six months ago. The suspected thief, an Italian named Luigi Valdarno, was found two days later in New York, shot dead but without the plans. Valdarno had been in a relationship with Elsa Hardt, who is believed to be an international spy working with Japan. The American Secret Service have since been tracking Hardt and several Japanese gentlemen in Washington. It now seems that one of the gentlemen and Hardt fled to England. Hardt's physical description matches that of Mrs Robinson.

At midnight, Poirot and Hastings enter the Robinsons' flat, where they wait and apprehend an Italian who is breaking in. Having disarmed the intruder, they all travel to a house in London, which Poirot has verified as Hardt and her accomplice's new residence. Poirot concludes that the pair had initially rented the Knightsbridge flat under the name of Robinson, but moved out upon discovering that an Italian gang was targeting them in revenge for the death of Valdarno. They sub-let the flat cheaply to a real couple of the same name whose wife roughly matched Elsa's description, in the hope that they would be killed in the spies' place. Hardt reveals the hiding place of the plans. Japp arrests the spies, and an agent of the Secret Service recovers the plans.