In this tale of a killer canine, man's best friend turns into his worst enemy. When sweet St. Bernard Cujo is bitten by a bat, he starts behaving oddly and becomes very aggressive. As Cujo morphs into a dangerous beast, he goes on a rampage in a small town. Stay-at-home mom Donna gets caught in Cujo's crosshairs on a fateful errand with her son, Tad . Stuck in their tiny car, Donna and Tad have a frightening showdown with the crazed animal.