Mild-mannered radio executive Brad Whitaker (Will Ferrell) faces immense challenges as he strives to be a good stepfather to his wife Sara (Linda Cardellini)'s two children, Megan and Dylan. Adding to his difficulties, Brad believes he is sterile due to a prior accident involving his testicles. As time passes, both children begin to warm up to him: Dylan confides in Brad about being bullied at school, while Megan asks him to take her to an important father/daughter dance.
Things take a turn when the children's biological father, Dusty Mayron (Mark Wahlberg), calls unexpectedly to announce his visit the next day, which leaves Sara feeling anxious about Dusty's presence in their home. In an effort to maintain stability for the kids, Brad persuades Sara that establishing clear boundaries is necessary between him and Dusty.
Upon Dusty's arrival, Brad is immediately unsettled by Dusty's charming looks and muscular build, as he easily bonds with Megan and Dylan. Despite Sara's reservations about Dusty's true nature, Brad attempts to be accommodating. However, it soon becomes evident that Dusty’s intentions are far from friendly; he is determined to push Brad out of his stepfather role and rekindle his relationship with Sara. Throughout the film, Dusty's antics—such as bringing a dog for the kids, completing a treehouse that Brad had been meaning to build with Dylan, and suggesting a visit to a fertility doctor—continue to undermine Brad's position.
The tension escalates, yet in a brief moment of camaraderie, Brad and Dusty team up to help Dylan defend himself, leading to a temporary truce. Additionally, the couple receives joyous news about Brad's improved sperm count, suggesting the possibility of expanding their family. However, Brad is blindsided when Dusty reaffirms his determination to uproot Brad and reclaim his former family.
In a desperate bid to win over the kids, Brad splurges on extravagant Christmas gifts, including a pony and expensive basketball game tickets. At the game, Dusty steals the show by showcasing his friendship with the team's coach, prompting an infuriated and intoxicated Brad to publicly lament about Dusty. This results in chaos, as Brad inadvertently injures a cheerleader and a child in a wheelchair with basketballs. Sara, overwhelmed with anger over Brad's behavior, asks him to leave. However, when Dusty steps in to comfort Sara, she firmly rejects him, instead demanding that he step up and fulfill his responsibilities as a father.
As days pass, Brad's despair deepens while Dusty grapples with his newfound role as a full-time dad. When Dusty contemplates abandoning his responsibilities just before Megan's father-daughter dance, his friend Griff (Hannibal Buress) encourages Brad to fight for his family. In a moment of courage, Brad confronts Dusty at the airport, where Dusty admits he is struggling with the difficult realities of fatherhood and expresses his admiration for Brad’s dedication.
This heartfelt moment leads both men to attend the dance together, where they learn that the bully Dylan has been encountering is a girl, resulting in an almost violent confrontation with her father. However, inspired by Brad’s parenting style, Dusty takes a different approach and diffuses the situation with an impromptu dance-off. This event encourages Brad and Sara to reconcile. Dusty, acknowledging his role as a co-dad, decides to embrace this new family dynamic.
In the end, everything falls into place: Brad and Sara welcome a new baby boy, named after Griff, and the children fully accept Brad as their stepfather. Meanwhile, Dusty flourishes in his career as a Panda DJ, gaining wealth and building a castle-like home across the street, signaling a newfound friendship between him and Brad. Dusty remarries a woman named Karen (Alessandra Ambrosio), who, like Dusty, has a daughter, placing him in the same situation he once created for Brad. Now, Sara finds herself feeling overshadowed by Karen’s achievements, anticipating the challenges of dealing with Dusty's exciting new family life.
