One of the things I judge these episodes by, especially the adaptations of the short stories, are the various extra touches they add that don't come from the original stories themselves. This episode distinguishes itself by having a barrage of extras, each one amusing in itself, but together, an ideal recipe for such a marvelous series.
The plot in itself is fairly minimal. Mr. Davenheim, a wealthy banker, walked away from his home one afternoon and has not been seen since. Suspicion falls on a business rival of his that visited the house on the day of his disappearance; there is some reason to believe that the rival has done away with Mr. Davenheim, but if so, where is the body? Japp makes a wager with Poirot: if he can solve the mystery in 7 days without leaving his apartment, Japp will give him 5 pounds. "Like robbing a baby", chuckles Japp as he leaves. But, not surprisingly, by employing Hastings as his substitute eyes and ears, Poirot arrives at the solution.
But the extras! The fact that Mr. Lowen, Mr. Davenheim's rival, is a budding race car driver, so that Hastings gets to visit the track, witness a race, and briefly sit in a race car himself. The very appropriate comparison of Davenheim's disappearance to a conjuring trick has Poirot spending the episode consulting The Boy's Book of Conjuring, and practicing a series of well-timed tricks all along. And the temporary addition to Poirot's entourage of a parrot allows for one of the best exchanges in the series (Poirot: Do not fraternize with that creature. I am still training him. Hastings: It's only a parrot. Poirot: I was talking to the parrot.) Genius! Another good line that was not an extra, but a slight variation on Christie's original story: Poirot, as he finally accepts the 5 pounds from Japp for having solved the mystery, says "Like robbing a baby". An excellent line to top off an excellent episode.