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. At the last minute Herbert sounds the alarm and reveals his identity. Polly, who is at the ball in disguise, is accused by Ruth Webster of participation in the robbery, but Polly unmasks Ruth as a notorious thief. After revealing her own identity as a newspaper writer, Polly is united with Herbert.