As with most screwball comedies, the premise of the film must first be accepted, since the entire film is a logical development from the original (zany) premise. In 'Monkey Business', Barnaby Fulton is working on the development of some kind of youth elixir, which he is testing on chimpanzees. Unbeknownst to him, one of his test subjects escapes his cage and successfully concocts the potion, leaving it in the water fountain. Of course, when Barnaby tests the potion on himself, he drinks some water to get the bitter taste out of his mouth--and almost immediately becomes about 20 years old mentally and physically. Before the elixir wears off, Barnaby gets a funky new haircut, coat and car, all with his boss' sexy young secretary (Marilyn Monroe, who else?) at his side. His wife Edwina (Ginger Rogers) then gets in on the action, taking some of the elixir to allow Barnaby to make scientific observations about someone else's reaction to it. It isn't long before she drags her husband to their honeymoon hotel, dances the night away, and impetuously starts divorce proceedings when he upsets her. The ending is a terrific exercise in belief-suspension, as the rejuvenated Barnaby and Edwina (simultaneously, this time) engage in paint wars, hair-pulling and scalping.
The best part of the film really would have to be the central performance given by Cary Grant as Barnaby Fulton. He's evidently one of Hawks's favourite actors, and for good reason too--he makes the trippiest of dialogue sound perfectly natural, and plays science-geeks and debonair reporters equally convincingly. With Barnaby, the viewer is instantly reminded of David Huxley, a role Cary Grant infused with life about 15 years ago in Bringing Up Baby. Just as David is kickstarted to life by Susan, Barnaby is youthened by the elixir, and in both films, it's a delight to watch the transformation take place. Initially, Grant's Barnaby is as stuffy as you can imagine a scientist--he's absent-minded and somewhat stern; in effect, all 'grown-up'. But the moment the youth elixir kicks in, the change is miraculous yet believable. Watch in delight as Barnaby flips an effortless cartwheel; drives like a daredevil; and conducts an entire chorus of children in a rousing war song. The 'joie de vivre' that Grant infuses his character with is almost palpable.
Cary Grant is also capably matched by Ginger Rogers in their second film together. Her ability to turn into a little girl is charming in the extreme, and you can see the years drop off her in her final stint as young Edwina... it's so evident that she's having fun as she tap-dances through the hotel, or flips rubber bands at people, chews gum, and scribbles "Barnaby loves Edwina" across the conference room chalkboard.
In general, the film itself is a little uneven: it has brilliant and hilarious moments, but you definitely get the feeling that much of the film is coasting on the considerable energy and skill of its cast--a splendid Cary Grant, a lovely Ginger Rogers, and an intriguingly young Marilyn Monroe. You probably won't be in too much of a hurry to rewatch this film once you've seen it the first time, but there's really no reason to put off your first viewing... so what are you waiting for?