Based on Emile Zola's novel, Therese Raquin, In Secret tells the story of Therese (Elizabeth Olsen), a woman married off to her first cousin, Camille (Tom Felton), by his controlling mother, Madame Raquin (Jessica Lange), after growing up together. Her father had abandoned her when she was just a young girl with her aunt, and never returned. When they move from the countryside to Paris, where she works at the beck and call of the Madame in a little shop on a dingy side street in the city, they meet a childhood friend of Camille's, the enigmatic Laurent (Oscar Isaac). He ignites a passion in Therese that she never knew existed, and they fall headlong into a mad affair that drives them to get rid of anyone who stands in their way. But to what end?