In this comedy, a bookish kid sister reads a psychology book and comes to believe that, according to the book, she is finally "grown up." To demonstrate her new maturity she begins chasing her big sister's fiance. She then winds up locked in her room during a party held in the fiance's honor. She manages to escape and begins impersonating a maid. She soon meets a handsome burglar who mistakenly believes that she is a moll. They end up attempting to burgle the fiance's home. Mayhem ensues until the young girl finally manages to steal the fiance's heart for herself.