My Lady's Latchkey (1921)

"She gave him her heart and her latchkey. Romance, Adventure, Mystery beckoned, and she responded."

Young Annesley Grayle, weary of a gloomy, uneventful existence with her aunt, accepts the proposal of a young American that she pose as his wife. Later, they are actually married. When, at a ball, she hears her husband accused of stealing a valuable diamond and realizes that he is indeed a thief, she hides the jewel to save him.