The movie Wicked Stepmother revolves around a suburban family led by a father named Jack. Jack's wife, Susan, dies suddenly, and he quickly remarries a younger woman named Miranda. Miranda is a beautiful woman, but her behavior towards the family is cold and cruel. She also insists on redecorating the house and throwing out all of Susan's belongings.
Soon after Miranda moves in, strange things start happening. Objects move on their own, doors slam shut, and the family's pet cat disappears. The family soon discovers that Miranda is actually a mother/daughter pair of witches who share one body. One of the witches, Jennifer, lives inside Miranda's body, while the other, Morgana, must live inside the cat.
As the witches wreak havoc on the family, Jack turns to his friend and private detective, Ralph, for help. They also enlist the help of a suspended police officer named Robin. Together, they try to come up with a plan to stop the witches and save the family.
Throughout the movie, the witches switch between living in Miranda's body and the cat's body. They use their magic to manipulate and control those around them, including Jack and his children. At one point, they even possess Susan's spirit and try to kill Jack.
In the end, the group of heroes manages to defeat the witches, and they are forced to leave the house. However, the film ends with a hint that the witches may still be out there, waiting to cause more trouble.