Poirot finds himself working on two cases, the first of which is more of an amusement, or so it seems. After meeting a delightful young couple, Stella and James Robinson, at a party he sets out to prove to Captain Hastings that there must be a reason for their good fortune in getting a lovely flat at a ridiculously low price. He is also brought into a more formal investigation by Inspector Japp who is hosting a loud-mouthed American FBI Agent who is on the trail of thieves who have made away with secret US Naval plans for a new submarine. The American isn't too impressed that Scotland Yard have brought in a private detective or that Japp has taken away his gun. When Poirot sees the man the FBI is trailing, he also realizes it's the same man who he has seen lurking outside the Robinsons apartment block.