A young couple, the Robinson's, rent a beautiful new flat for about the half the going rate of the same flats in the building. Poirot is puzzled by this, when he hears that many others had tried to rent the flat and were turned down. Poirot believes there is something unique about the Robinson's, and that whatever it is that makes them different, that difference is somehow related to a woman who stole secret American submarine plans and has fled to London. An FBI agent has joined forces with Inspector Japp and his team from Scotland Yard to follow the thief and recover the stolen plans, but the partnership between the FBI and Scotland Yard is far from cordial.