Posy Pinkerton needs to step out of her shell. For this late-20s serial monogamist, life has been smooth sailing. She's got her doctor fiancé Tom, her burgeoning career as a medical illustrator, and a side hobby as a visual artist. But after a chance encounter with Jack, a handsome and worldly businessman, Posy's shell begins to crack. She finds him mysterious and he finds her charming, and for Posy it is an opportunity to try on confidence for size. She is reluctant to take it any further than mild flirting, but her polar-opposite cousin Laura pushes her for one last fling before she settles down with Tom, her one-and-only. Posy's growing desire to break free allows her to exercise her inner self with Jack, an alter ego she's created called the Queen of Sin. As she's being swept up by Jack, she doesn't realize that she's really making a deal with the devil.