A young woman, Madeline, is happily married to Ernest, and their life seems perfect, until an old friend, Helen, returns and threatens to unravel their happiness. Jealous of Madeline's relationship, Helen is determined to kill her and steal her husband, a promise she made in a fit of jealousy. After recovering from a mental breakdown, Helen is set on fulfilling her dark promise, but things get more complicated when she discovers a mysterious potion that can turn her into Madeline.