Ever anxious to keep his little gray cells invigorated, Hercule Poirot takes on two cases which soon look as if they may collide.
At a party, he and Hastings meet the Robinsons, a young couple who brag about renting a dirt-cheap flat in a gorgeous apartment house in an excellent neighborhood. Poirot finds this suspicious and decides to investigate.
Then Inspector Japp calls him in on an espionage case. The U.S. Navy has had secret plans for a submarine stolen, and not only is Scotland Yard involved, but the FBI. Their representative is dismissive of Poirot (and also keeps claiming that there is no such thing as the Mafia). When Poirot sees the man they are tailing, he recognizes him as someone who has been standing outside the apartment house of the young couple.
Delightful episode, with Poirot again doing his second story man routine, complete with tools, as he picks a lock in the apartment while Hastings distracts its occupants.
It's hard to fault such a well-done series, but with so many years of Poirot, there are bound to be some episodes that are better than others. This is definitely one of the better ones.