Granny Get Your Gun (1940)

Minerva Hatton is back in Nevada, where she grubstaked her fortune years ago. Her granddaughter Julie Westcott is visiting while getting a divorce. They are blackmailed by Julie's husband, Philip, who has two gambling checks Julie has given a gambling casino. Minerva, trying to buy them back, comes across Philip's murdered body. Believing Julie guilty, she substitutes her fingerprints and pleads guilty. When she learns that Julie is innocent of the killing, she does an about face, appearing in court as a sweet old lady instead of her usual rugged desert outfits, to play on the jury's sympathy. Freed, she still must find the killer to clear Julie, to whom the evidence points. After a little detectice work, she is convinced of the identity of the killer. She has herself sworn in as a temporary sheriff, jumps into her Model-T Ford and uses her marksmanship to nab the killer.