"Poison Ivy" is a thriller that starred a young Drew Barrymore as the title character, Ivy. The movie also starred Tom Skerritt and Cheryl Ladd as Darrel and Georgie Cooper and Sarah Gilbert as their daughter Sylvie Cooper. Sylvie and Ivy meet by a riverside and despite Ivy's poor appearance, she is very intelligent. The girls are interrupted by a boy who tells them that a dog has been hit by a car and the dog is hurt. They go over to check on the dog who is obviously suffering. As Sylvie is kneeling by the dog, Ivy sadistically smashes the dog in the head with a pipe to end its suffering.
Ivy tells Sylvie that she suffers an uncomfortable home life and Sylvie introduces Ivy to her parents. Sylvie's mother, Georgie, is very sick. After initially being weary of Ivy, she earns their trust by helping around the house. Ivy spends a lot of time at the Cooper's home. She shares Sylvie's room and spends time with both parents.
As Ivy grows closer and closer to the family, things start to change. Ivy volunteers to help Darrel with his work and to help plan a party to help his business. Ivy seems to morph into Georgie's parental role. She changes her hair and makeup to resemble Georgie and starts to borrow from her wardrobe. Sylvie starts to suspect that Ivy has some dangerous personal issues and emotions.
Georgie is despondent. She confides to Ivy that she has thoughts of suicide. Ivy makes a sudden violent turn and actually pushes Georgie off of her balcony; killing her and making it appear as a suicide.
The family's world turns upside down and Ivy is attempting to take over. She makes an attempt to seduce Darrel. Sylvie investigates and finds out about Ivy's intentions and her actions. Ivy faces off against Sylvie in a bitter battle to end the film.