Brad Whitaker (Will Ferrell) is married to Sara (Linda Cardellini) and is stepdad to her two kids Dylan (Owen Vaccaro) and Megan (Scarlett Estevez). Brad is unfortunately unable to produce children of his own due to an incident at the dentist where his groin was exposed to x-ray radiation. Despite his best intentions and hardest attempts, the kids don't really take kindly to Brad. Still, his biggest hope is for them to see him as an acceptable father figure.
Over time, the kids warm up to Brad. Dylan even confides to Brad that he is being picked on at school by some 4th graders. Brad gets excited since he feels he's built trust between him and Dylan (even though he goes ahead and tells Sara). Megan invites Brad to the daddy-daughter dance, which makes Brad so happy that he cries.
The family is watching "The Princess Bride" when the phone rings. Dylan answers and becomes ecstatic to hear his biological dad Dusty's (Mark Wahlberg) voice. Megan runs and grabs the phone too. They pass the phone to Brad, who agrees to pick up Dusty from the airport the next day. Sara gets worried since Dusty is just showing up out of the blue, but Brad is confident that he can handle him.
Brad goes to pick up Dusty at the airport and is stunned to see a much more handsome and cooler-looking guy descending the escalator. Brad approaches the man and asks if he is Dusty, but he says no. When Brad gets home, he sees a motorcycle parked outside. Dusty is already in there playing with the kids. He formally meets Brad and comes off as friendly. Later, when Dusty tucks the kids into bed, he is obviously trying to look better than Brad by giving the kids $20. The men go outside and share a beer, and they drop the nice acts to say what they're really thinking. Brad asks Dusty why he's there, and he says he wants to be a good father figure now that Sara has remarried, but he promises to let the kids accept Brad as their new dad.
Dusty starts the pissing contest by getting the kids a mangy dog that he calls Tumor ("because he grows on you"). The dog doesn't like Brad. Brad takes Dusty to his job at a radio station where Dusty meets Brad's boss Mr. Holt (Thomas Haden Church). The two quickly hit it off, and Dusty even scores a gig as the voice of their radio station mascot, earning him residual pay.
Brad is asked to move Dusty's motorcycle from the driveway so Sara can get her car out. Brad lied about knowing how to ride, so when he tries it out, he rides hard into the house, nearly hitting Sara and the kids, and ends up getting thrown into a wall while the motorcycle crashes out the house and onto Brad's car. Dusty punches a hole in the wall to get Brad out. Sara later calls a handyman named Griff (Hannibal Buress) to fix the damage. Dusty tells Brad to fire him since they can fix it themselves. Dusty uses both incidents to teach the kids lessons working against Brad's favor. Brad fires Griff and is mistaken for being racist.
Dusty brings Griff back to make himself look good and to also fix the house while also building a treehouse in the back, along with a skate ramp, where Dusty shows off his moves. Brad, not wanting to be outdone, pulls out his old skateboard and attempts to do some moves, only to fly into the electrical wires and get electrocuted. Dusty prolongs Brad's pain to look heroic in front of the kids before he does CPR to revive him.
Dusty gets into Brad's head by mentioning that Sara wants another baby, knowing Brad can't give her one. When he mentions it to Sara, she admits that she does want another baby, but she is still happy with Brad. They go get a check-up from Dusty's buddy Dr. Francisco (Bobby Cannavale) to see whether or not Brad can truly get Sara pregnant. This gives Dusty a good opportunity to show off just how much nicer his member is compared to Brad's. Brad later gets the news that he has a good chance of procreating successfully, to Dusty's dismay.
Dylan comes home crying after getting bullied again. The men try to show the boy how to defend himself. Brad suggests dancing as an alternative, but even Sara wants Dylan to fight back. Brad talks about when he got bullied and lies about having punched a kid to get him to stop bothering him, but he admits the truth to Dylan and says he never fought anyone. This inspires Dylan to fight back so that he doesn't end up a wimp like Brad.
Brad decides to step up his game to win the kids' approval by decorating the house with Christmas stuff, giving the kids a bunch of presents, like a pony for Megan, and tickets to a Lakers game, since they're Dylan's favorite team. Brad takes everyone to the game, but gets Dusty a seat away from him, Sara, and the kids. However, Dusty knows the Lakers' coach and even gets him to introduce Dylan to Kobe Bryant. Brad gets upset, but starts to flip out when he learns that Megan asked Dusty to go with her to the daddy-daughter dance, leading him to yell in front of everyone else and admit to Sara that he bought the tickets off a scalper for $18,000. Everyone but Brad gets to sit courtside. Brad drinks a bunch of beers and wins a chance to make a shot to win the family a vacation. Too drunk, Brad talks into the mic, announces he had sex with Sara to the whole arena, and then throws the ball and hits a cheerleader in the head. He grabs another ball and hits a kid in a wheelchair. He is taken out by four guys at once.
Brad packs his stuff and is kicked out of the house, leaving Dusty to be the dad. Brad stays in his office while Dusty takes on parental responsibilities. It proves too much for Dusty, and he decides to bail and skip town. Griff goes to Brad's office and encourages him to return home and make it to Megan's dance.
Brad rushes to find Dusty at the airport. Dusty admits he can't be a good parent, but Brad decides the two need to work together so they can both be good dads for Dylan and Megan. They make it to the dance, where Sara isn't happy with either of them. The men go to dance with Megan, only for her to tell them that Dylan is getting bullied by the 4th graders...all of whom are girls. Dylan punches the main girl in the face, kicks her in the groin, and calls her a bitch. Her dad (Bill Burr) gets involved and tries to fight the men. He punches Dusty, and Dusty gets ready to fight back, only to look around at the other girls and decide to dance instead, leading everyone else to continue dancing. Dylan ends up dancing with the 4th grader when everyone realizes that she was picking on him because she likes him.
A while later, Brad and Sara have a baby of their own. Dusty has built his own home down the street with his residual pay and has gotten married to another woman named Karen (Alessandra Ambrosio), whom Sara is a little intimidated by. Dusty is trying to win Karen's daughter's approval. They receive a visit from Karen's ex, Roger (John Cena), who is twice as big as Dusty. Dusty immediately feels what Brad felt when he first saw Dusty. Dusty goes to greet Roger, only to get the cold shoulder.
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