There's no denying it: Song of the Thin Man (1947) is the weakest entry in the series. It has several problems, the primary one being that the mystery is just a so-so one that progresses in an often heavy-handed manner, then resolved when Nick sets up an equally heavy-handed deus ex machina to trick the killer into giving himself/herself away.
The plot is potentially good. A band leader is murdered aboard a gambling ship. He was a ladies man with an eye for women both married and unmarried AND he owed a gambler $12,000 dollars, so there's plenty of motive to go around. Nick and Nora become involved when the main suspect asks for their help. All perfectly good stuff with an interesting setting. But the script feels as if it needed one or two more re-writes to polish it up--events play out in a contrived manner rather than flow naturally.
Also, Nick Jr. (played by a young Dean Stockwell) hampers the Charles' style. With a growing son, the couple is more domesticated than ever. In real life, of course, this would have been admirable and appropriate. But in a universe in which married couples frequently stumble over corpses and then try to solve the crime, domestication is never a good thing. We don't want to see them raising a kid and--even though those scenes are done with a fair level of humor--they are disconnected from the murder mystery plot and add nothing to the overall film.
But the movie still manages to be entertaining. This is due in part to the fact that its impossible to watch William Powell and Myrna Loy play off each other and not have enjoy yourself. The supporting cast, including Keenan Wynn as a band member who gets roped into helping the
Individual scenes stand out as well, such as Nick and Nora trying to grasp the meaning of Keenan Wynn's hep slang. Or the scene I've included here in which the Charles' get an awful lot of information out of a hotel clerk who claims he never snoops on his guests. Also, one character's reaction when the killer is revealed adds a sudden dose of effective drama.
My understanding is that William Powell felt he was getting too old for the role of Nick and wanted to end with series after this film. Perhaps he was right--as Nick and Nora got older, the feel of the movies would have inevitably changed. (Though Myrna Loy was still as much of a goddess as always in the looks department.)
On the other hand, had Song of the Thin Man had a better script (and if Nick Jr. were shipped off to military school), perhaps they could have continued the series longer without a loss of quality.
We'll never know. But at least we got to spend six movies with Nick and Nora. They were indeed a fun couple. If more married couples regularly discovered dead bodies and solved murders, the real world would be an infinitely more interesting place.
So that's it for the Thin Man films. Next, we're going to stick with unusual detectives and look at the first three Hildegard Withers films from 1932 to 1935 (the three that star Edna May Oliver as Hildegard). Hildegard was a spinster school teacher with a knack for solving crimes, so she can't help but be interesting.