A young boy stays with his aunt and uncle, and befriends his cousin who's the same age. But his cousin begins showing increasing signs of psychotic behavior.
Although based on a novel by Lionel Shriver, it seems to be a direct descendant of The Bad Seed, the 1956 thriller with Patty McCormack's Oscar-nominated turn as a murderous moppet (let's not waste time on 1993's similarly themed The Good Son; as a homicidal brat, Macaulay Culkin was about as menacing as an inchworm).
(His pathological coldness recalls Macaulay Culkin’s character in “The Good Son.”) Kevin’s idea of playing with his younger sister (Ursula Parker) consists of turning the vacuum cleaner on her hair.