Gary Bloom's life is turned upside down when he weds Samantha, who comes with a 5-year-old son, Lucas. Despite his best intentions, Gary finds it challenging to connect with the quiet and introspective boy, whose gaze often feels like it passes right through him. Things take a strange turn when Karl, his wedding videographer, calls him with a cryptic warning about something extraordinary hidden in the footage, a call that Gary brushes off as trivial.
As Gary's focus shifts to an old nunnery that he owns, he discovers that it is now inhabited by Father J.D. Gospel, a charming leader of a mysterious doomsday cult. This change seems to trigger a series of cosmic events, leading to a shocking revelation from Lucas' school principal that the boy's erratic behavior has resulted in a tragic incident involving a teacher. With the community buzzing over this calamity, Gary is thrust into the role of scapegoat, facing suggestions to seek therapy himself for what is perceived as the root of the chaos.
Samantha struggles with the fallout, feeling the weight of blame directed at Lucas and Gary. Yet, Gary is determined to reassure her of his unconditional love for both of them, even as fissures begin to form in their relationship. In a group therapy session that includes his friend Al, Gary lays bare his anxieties about Lucas, revealing his fears among others who nod in solidarity, sharing their own concerns about the complexities of fatherhood.
As bizarre occurrences escalate, a particularly chaotic moment occurs at Lucas' birthday party when a clown suddenly ignites in flames. Left reeling, Gary begins to suspect that Lucas is somehow linked to the mounting chaos. However, it's Karl who delivers a shocking disclosure: during their wedding, Lucas was miraculously untouched by a devastating tornado.
This revelation sends Gary spiraling into a deep investigation, uncovering Samantha’s shadowy past and the unfortunate fates of her exes. The sole survivor, Gabriel, becomes a critical lead that guides Gary to uncover the dark details of Lucas’ conception, tied to a sinister cult ritual.
With the stakes raising perilously high, Gary persuades Al to embark on a dangerous quest to Bethlehem, Pennsylvania, where they meet Gozamel, a demon hunter whose mission is to thwart an impending apocalypse. Realizing the gravity of their mission, they find themselves in the unimaginable position of needing to kill Lucas with the fabled Knife of Destiny to avert the end of humanity. This solemn duty intensifies when Gozamel meets an untimely death in a tragic car accident, leaving Gary burdened with the life-or-death decision concerning his stepson.
Upon returning home, Gary walks into a scene that is equal parts comforting and chaotic: Samantha and Miss Shaylock, a Child Protective Services representative, stand together like beacons of reason amid the turmoil. Their plea for Gary to take Lucas to bed leads to a whirlwind of events. The night devolves into chaos as Lucas, embodying innocent mischief, buries Gary in the sandbox, initiating a series of comedic yet poignant moments that illustrate the complexities of family life.
As Gary's mental state further deteriorates, he becomes convinced that Lucas embodies pure evil—the Antichrist himself. This belief leads Gary to orchestrate a tragic accident during a trip to Waterland, a Pope-blessed amusement park, where he instead finds poignant moments of joy and laughter with Lucas, further complicating his initial plans.
In his growing desperation, Gary attempts to rid the world of what he perceives as evil by equipping Lucas with floaties and sending him down a water slide, believing it to be his doom. However, fate intervenes as Lucas's supposed "death" is met with a celestial sign spelling out "Love" across the sky. In a moment of clarity, Gary snatches his son back from the edge and treats them both to ice cream, resolving their earlier hostilities with heartfelt apologies.
Yet the drama amplifies with an amber alert for Lucas lighting up Gary’s phone, summoning the authorities to take his child. To compound the crisis, Miss Shaylock, revealed as a staunch follower of Father Gospel, kidnaps Samantha and vanishes into the night. With the weight of these events closing in, Gary finds camaraderie among his fellow stepdads, who rally behind him in a mission to rescue Samantha and Lucas from the clutches of Father Gospel's nefarious cult.
This adventure leads them to the nunnery where Father Gospel and his followers are preparing to carry out a sacrificial ritual on Lucas to bring forth the apocalypse. Gary’s frantic efforts yield a narrow escape, but not before Lucas accidentally unleashes a portal to Hell, threatening to unleash unimaginable horrors.
As Samantha takes decisive action and uses her maternal instincts to cast Father Gospel into the abyss, a sense of calm returns to their tumultuous lives. Weeks later, Gary and Lucas stand proudly together at the Okatok Soap Box Derby, a vibrant testament to their unbreakable bond as father and son, finally able to embrace each other without the burden of doom hanging over their heads.
