In the movie The New Stepmother is My Ex, Joon-soo is suddenly dumped by his girlfriend Sung-eun and is surprised when she marries his father, becoming his new stepmother. This is unacceptable and Joon-soo is heartbroken until a mysterious woman named Minki appears before him. She knows about him and his biological mother. She tells him that good things will come to him and leaves after giving him a photo frame with a cat.
A sexy woman appears the next day and comforts him, making his stepmother jealous again. Will Joon-soo get over his ex-girlfriend? Or will they get back together.