In the film, a man suspects that his new stepson is "a little evil", after realizing that the boy may have caused two mysterious suicides. While investigating the boy's past, he is informed that his stepson is the Antichrist and that he may have to kill the boy to prevent the end of the world.
Plot
Gary Bloom marries Samantha, who has a 5-year-old son, Lucas. Gary struggles to connect with the quiet Lucas, who ignores him. Gary receives a telephone call from his wedding videographer Karl, warning that something very unusual is in the footage, but Gary is uninterested. Gary stops by one of his properties for sale, an old nunnery, where Father J.D. Gospel, the leader of a Doomsday cult, buys it on the spot. Gary is summoned to Lucas’ school where the principal informs him that Lucas spoke out of turn in class and told his science teacher to “go to hell”, after which she killed herself by jumping out the window and got impaled on a fence. A psychiatrist stresses that Lucas see a counselor and Gary, apparently the main source of Lucas’ erratic behavior, should do the same.
Samantha is upset by the news and feels that everyone, including Gary, is unfairly blaming Lucas, but Gary assures her that he loves them both. At therapy, Gary confides to the other stepfathers, including his friend Al, that he thinks his stepson might be a little evil, and they all sympathize. At Lucas’ birthday party, a clown lights himself on fire, and Gary is led to believe Lucas is responsible. Karl shows Gary the wedding video, revealing a possessed-looking Lucas untouched by a tornado. Karl tells Gary that all Samantha's previous boyfriends are dead except Gabriel, giving Gary Gabriel's address.
When Gary inquires about Lucas' biological father, Samantha reluctantly admits that years earlier, she was part of a cult and Lucas was conceived during a ritual. Gary convinces Al to help him, and they find Gabriel in the basement of a church where he is self-flagellating. Gabriel reveals that Lucas is the Antichrist and tells them to travel to Bethlehem to find Gozamel the demon hunter. Gary and Al view a television news report on rioting due to the Apocalypse in Bethlehem, Pennsylvania, realizing that is where they need to go. They find Gozamel, who informs them they must kill the child with the Knife of Destiny to prevent the end of the world. Gozamel is killed in a car accident, but not before he gives Gary the Knife of Destiny to kill Lucas on hallowed ground.
Gary arrives home to find Samantha with Miss Shaylock, a woman from Child Protective Services. They encourage Gary to put Lucas to bed, but this goes horribly wrong, ending in Lucas burying Gary in a sandbox. Samantha digs him up, taking Lucas’ side for being a confused kid, and Gary screams that Lucas is the Antichrist. Samantha is devastated, and Gary apologizes. Convinced Lucas is the Antichrist, Gary takes him on a trip to Waterland, an amusement park blessed by the Pope, intending to drown Lucas in what would appear to be an accident, but Lucas and Gary actually start to bond as father and son. Gary reluctantly equips Lucas with sand-filled floaties and sends him down a water slide to his death, but sees the word "Love" in the sky and takes it as a sign. Gary saves Lucas, taking him for ice cream, and they both apologize for trying to kill each other.
An amber alert for Lucas appears on Gary's phone, and police arrive to arrest Gary as Miss Shaylock, revealed to be a disciple of Father Gospel, takes Lucas. Father Gospel also kidnaps Samantha. Gary escapes to save her and Lucas, with help from the other stepdads. They travel to the old nunnery where Father Gospel and his disciples prepare to kill Lucas and bring about the end of the world. When Lucas opens a tunnel to Hell and starts to fall into it, Gary saves him. Once freed, Samantha knocks Father Gospel into the tunnel. Weeks later, Gary and Lucas race the Okatok Soap Box Derby, finally happy as father and son.
Cast
Adam Scott as Gary Bloom, Samantha's husband and Lucas's stepfather
Evangeline Lilly as Samantha Bloom, Gary's wife and Lucas's mother
Owen Atlas as Lucas, Samantha's son and Gary's stepson
