A mystery writer gets her ex-husband to solve the murder of a jockey by using his medical skills and amateur detective work.
Paula Bradford (Jean Arthur) sues Dr. Lawrence Bradford (William Powell) for unpaid alimony and stays for dinner, wanting to marry again. Mike North agrees with her that a jockey was murdered and asks Bradford to do an autopsy exam. Bradford receives a package of money for North and replaces the cash with papers. Paula moves in to protect her ex-husband. Bradford gets a call to leave but goes back and is shot at by a man looking for the money. They fight, and Paula breaks a vase on Bradford's head. At the track bookmaker Nick Martel (Robert Armstrong) tells Bradford that North is a double-crosser. Paula takes Bradford's scalpel, and analysis shows gelatin. Bradford calls Inspector Corrigan, who is on his way. Bradford answers the doorbell to find a man dead with a note that he wouldn't turn over the money either. Corrigan (James Gleason) questions and suspects Bradford but does not arrest him. Bradford learns the coroner found the left ventricle collapsed.
Paula says Hutchins called North four times. Bradford leaves to question Hutchins, who says Henry Strand froze him out of a deal with Summers. Bradford questions Strand and then John Summers (Grant Mitchell) and Mrs. Summers (Erin O'Brien-Moore), who calls on him to admit she wrote the threatening note to the jockey. Murphy is trailing Mrs. Summers, who is paying him not to. Murphy says the other man is Nick. Bradford gets Murphy to open Nick's safe for him; he looks at the books, but Nick comes in with his men. Bradford is saved by a pre-arranged phone call from Paula which he pretends is from Corrigan. Nick says North's winning $25,000 bet at 4-1 was made by a man with a scar and gives Bradford the address. Bradford sends himself a telegram from North to go there and finds the man with the scar impersonated North but is dead. Someone shoots Bradford from the window.
Papers report that Bradford was killed; but when Paula says good-bye, he asks her to marry him. Corrigan tells them the man with the scar died just like the jockey. Bradford explains they were killed by black widow spiders in gelatin capsules. North was killed for investigating, as he himself almost was. They expect another murder at a similar race; but Bradford inoculates the jockey and has cameras film the jockey before the race. The jockey wins before he collapses, and Bradford treats him. All the suspects are summoned, and Bradford shows them the film, which causes Summers to pull a gun; but Bradford and Martel jump him, as Paula hits Bradford with a vase again. In the final scene Bradford marries Paula.
Charm and wit raise the entertainment value of this far-fetched mystery, since black widow bites are rarely fatal. The audience is distracted from their own problems trying to figure out the complicated plot.