The plot here is fairly straightforward: Richard Carewe (Conway Tearle) has raised Richard "The Imp" Dane (David Manners) in his father's stead, and has arranged for him to marry Phyllis Ericson (Loretta Young), but whilst out on a tear (under the premise of attending a psychology lecture) he becomes smitten with a nightclub singer named Kara (alias The Firefly, played by Myrna Loy). The Imp convinces Kara that he is a rich young bachelor and marries her in secret. When Phyllis discovers a letter from Kara to The Imp, Richard the Elder deftly convinces her that the letter was actually written to him -- the letter having been addressed only to "Richard" -- and sets about trying to fix the whole mess. He bribes Kara into pretending that she and he are in love, and shenanigans ensue.
It's a standard Good-Woman-Bad-Woman motif, with Loretta Young as the innocent sweetheart and Myrna Loy as the money-grubbing chiseler. Loretta gets top billing and does a good job with her character, but as she is wont to do Myrna basically steals every scene in which she appears, and by and large steals the whole movie -- in one particularly memorable scene she throws a delightful little fit, replete with slapping of man-face and tossing of pottery (this after The Imp admits that he isn't all he claimed to be). Myrna also sings two numbers in the nightclub, although it's fairly obvious to the attentive viewer that it isn't her voice, and that she is only lip-syncing. Still, she sells the hell out of it, bless her heart.
Loretta and Myrna appear in only a single scene together, but it is easily the best scene in the movie. Phyllis follows Richard to the nightclub and pretends to be starstruck by Kara, essentially telling Richard that she wants to be just like Kara when she grows up, much to his consternation (she suspects that Richard's courtship of Kara isn't on the level, as the saying goes, and it's clear that her praise is purely to test Richard).
The male leads, on the other hand, aren't nearly so interesting. Conway Tearle does a decent job with his character, but there isn't much about his role that's particularly memorable. The same goes for David Manners, whose character's nickname might be the only really notable thing about him -- he doesn't do much to break the mold of the Generic 30s Guy, although it should be said that the script doesn't really give him much opportunity to do so.
Finally, there's the ending. Although Richard manages to get The Imp to divorce Kara, he refuses to marry Phyllis, claiming that he never actually loved her. Phyllis takes the news shockingly well, exclaiming that she really loved Richard the whole time. This ending really doesn't make much sense at all, which has the unfortunate effect of upending the picture in the last five minutes. We do see that The Imp's devotion to Phyllis is questionable, but the movie gives us no hints whatsoever that Phyllis is really in love with Richard. It's kind of a shame, since I really enjoyed the picture up until that the script decided to drop that pipe-bomb in my lap.
Yet even if the ending doesn't really work, the picture is still enjoying. The acting feels quite crisp for its time, and the camera work here isn't nearly as stiff as in other movies from the same time. Then, of course, there's Myrna Loy, who has an uncanny ability to make any movie better by here mere presence. Indeed, she's pretty much the number one reason to watch the picture. The Truth About Youth is worth at least one viewing, especially for Myrna Loy fans.