The film opens two years after the events of the first movie. Dan Morgan (Mark Wahlberg) and his wife Jessica (Michelle Monaghan) have moved their family to a seemingly safe, technologically advanced suburban neighborhood in Montana, determined to live a completely anonymous and boring life. Dan is now selling high-end solar panels, while Jessica is running an organic food delivery service. Their children, however, are growing tired of the constant vigilance and the suffocating secrecy.

The conflict is initiated when their teenage daughter posts a innocuous photo online a selfie that accidentally captures a specialized, military-grade router Dan installed. This image is intercepted by "The Successor" (Kit Harington), a meticulous, highly organized former asset of the shadowy organization Dan used to work for. The Successor views Dan's peaceful retirement as a profound act of disrespect and vows to dismantle his perfect life, one family member at a time.

The Successor, who runs an ultra-exclusive international security consulting firm as a front, sends a series of highly trained, stealthy operatives disguised as utility workers, neighbors, and even a new soccer coach, infiltrating the Morgans' new life. Dan and Jessica immediately recognize the signs, forcing them to activate their newly established "Plan B," which involves turning their smart home into a tactical bunker disguised as a domestic nightmare.

The tension builds as Dan and Jessica must navigate a deadly game of cat-and-mouse during a routine week: fighting off highly trained assassins during a PTA meeting, using their solar panel truck as a getaway vehicle, and defusing a bomb disguised as a high school science fair project. The humor comes from the juxtaposition of lethal action and mundane suburban responsibilities, with Dan struggling to attend a parent-teacher conference while bleeding from a knife wound.

The mid-point twist reveals that The Successor's true target is not Dan, but Jessica. He believes Jessica was responsible for the downfall of his family years ago during an overlapping mission. This revelation forces Jessica to confront her own buried history and take a more central, aggressive role in the counter-operation, proving her skills are equal to, if not superior to, Dan's.

The climax is set during the annual neighborhood block party and chili cook-off. The Successor takes the family hostage in their own home, setting up a final, theatrical confrontation. Dan and Jessica must coordinate a desperate, improvised attack, utilizing everything from lawn mowers and barbecue tongs to their children's forgotten spy techniques. The children, realizing the full truth of their parents’ past, step up to help secure their freedom.

The film concludes with The Successor defeated and the family standing together, fully exposed but stronger than ever. They realize they can never truly be anonymous. Instead of running again, they make a new "Family Plan": they will remain in their house, keeping their tactical readiness high, deciding that if the world comes for them, they will be ready to fight back as a family.