The Perfect Sap (1927)

Wealthy young Herbert Alden, a would-be detective, is practicing housebreaking with his valet, an ex-convict, when he meets George and Polly, two real thieves; they are discovered and, following a chase, they go to Herbert's city apartment. George arranges with one of them, Tony-the-Lizard, to rob the guests at a ball given by Herbert's father at his country home. Herbert discovers that Tony is Tracy Sutton, a social lion engaged to Herbert's sister Roberta; and believing Herbert to be a famous criminal, Tony seeks his advice.