Dr. Barnaby Fulton (Grant) is a chemist at a pharmaceutical company, trying to develop an elixir of youth. While he means well, his extreme absent-mindedness combined with his dedication to his work often cause him to neglect his wife Edwina (Rogers). Their relationship hasn't slid into Dead Sparks, but there's an element of dull routine as the two advance into middle age.
That is, until one day when one of the lab chimpanzees watches Barnaby fiddling with his flasks and beakers, trying to find the combination which will give him the youth potion. The monkey escapes from its cage, starts fiddling with the flasks and beakers, and winds up creating what turns out to be the real youth potion—namely, one that restores vigor and vitality and fixes stuff like back pain or bad eyesight, but doesn't actually make the person physically younger. Then the monkey dumps the potion, into the water cooler. Various madcap antics ensue.
Marilyn Monroe, then just on the cusp of superstardom, plays Miss Laurel, secretary to Mr. Oxley (Coburn), boss of the pharmaceutical company.
- An Aesop: Delivered by Barnaby in the last scene. "You're only old when you forget you're young."
- Cowboys and Indians: Barnaby runs across some neighborhood kids playing cowboys and Indians. Given that he has mentally reverted to elementary school age, Barnaby joins them. Then he and the kids get revenge on Hank.
- Ditzy Secretary: Laurel is extremely dumb. When Barnaby notes that she's at work early, she says "Mr. Oxley's been complaining about my punctuation, so I'm careful to get here before nine." Then there's the scene where Oxley tells her to go find Barnaby, who is off looking to buy a new Ford sedan.Oxley: Miss Laurel, now listen carefully! I want you to go to every Ford agency in town, and find Dr. Fulton!
Laurel: But Mr. Oxley, which shall I do first?
- Fanservice: The one scene where Miss Laurel is not wearing a tight sweater comes when she and Barnaby go to a swimming pool and Laurel puts on a swimsuit.
- High-Class Glass: Oxley is marked as rich and snobby by the monocle he wears.
- Medium Awareness: A gag in the opening credits. The title comes up over a shot of Barnaby and Edwina's front door. Cary Grant steps out. The offscreen voice of Howard Hawks says "Not now, Cary," and Grant goes back inside. Some more credits go by, Grant comes out again, and Hawks says "Not now, Cary" again. Finally the credits end and the movie starts and Barnaby finally gets to come through the door.
- Mischief-Making Monkey: All the comedy is kicked off by one of the chimps getting out of its cage and mixing Barnaby's chemicals together.
- Romantic False Lead: Hank the lawyer, who at some point in the backstory was rejected by Edwina in favor of Barnaby. He's still sniffing around, and he only too eagerly dives in when the effects of the drug lead Edwina to call him.
- Screwball Comedy: A lot of silliness ensues when first Barnaby, then Edwina, then both of them together take does of the drug that first regresses them to teenagers and then to children.
- Sexy Secretary: Miss Laurel, played by Marilyn Monroe in a series of tight skirts and sweaters. In one scene Oxley gives her a letter and says "Find someone to type this." After Laurel says that she wants to try, Oxley says that it's very important, so she needs to get someone to type it for her. As Laurel sashays back out of the office, Oxley murmurs to Barnaby, "Anyone can type."
- She's Got Legs: Laurel demonstrates how much she likes the stockings Barnaby invented by hiking up a skirt and showing a long, lovely leg.
- Squirrels in My Pants: Ingesting the formula causes Edwina to get silly, as demonstrated when she pulls a fish out of the fish tank and drops it down Mr. Oxley's pants.
- Sweater Girl: This trope is really pushed to the limit with the absurdly tight tops that Marilyn Monroe wears throughout the movie (except for the scene where she's in a swimsuit, that is).
- You No Take Candle: Some racist humor when Barnaby becomes an "Indian" in the cowboys and Indians game, and starts chanting stuff like "Me wanta wampum."