When we meet Eileen, we see her sabotaging her big sister's wedding. We find out that she blames her older sister, Claire for a car accident that has left her deaf and mute. As Eileen tells us the story from her perspective we see a series of flashbacks that tell us about a love affair that she had with her sister's groom James. They had kept the relationship a secret due to an age difference, from everyone except Claire. When James won't believe Eileen's story about Claire trying to murder her, she sets her plan into motion and swears revenge on the newlyweds. As we watch her try to ruin Claire's big day, the audience is left guessing if Eileen's story is the truth.