A friendly St. Bernard contracts rabies and terrorizes a small town.Frustrated Donna Trenton is a house wife whose life is in turmoil after her husband finds out that she has had an affair. Brett Camber is a little boy whose only companion is a St. Bernard named "Cujo", who in turn is bitten by a rabid bat. Donna's husband Vic on business, and thinking about his marital troubles. Donna and her 5 year old son take her car to Brett's dad's car shop... the car fails, and "Cujo" is very, very sick and becomes very violent.