The Good Sister follows a couple who is working to put their crumbling marriage back together, when the wife receives a letter informing her that she has a long-lost twin sister. First thinking that the new family member will bring the couple closer together, the husband then begins an affair with the sister and discovers that she is treacherous and dangerous.

A devoted single mother, Constance, struggles to balance her newfound love with Leo and the obsessive, destructive behavior of her son, Anthony, who will go to extreme lengths to keep her to himself.