At a business meeting while seated about the table, Charlie (John Travolta) is telling an embarrassing story about his partner, Dan (Robin Williams), from years ago. Apparently these childhood friends grew up, went to college, and started a sports marketing business together, and are very successful. Charlie has never been married, but Dan is divorced, after a 14 year marriage and no children. The story goes that Dan was so depressed that ladies-man Charlie took him for a weekend of fun after his divorce, to cheer him up, to Miami. They got drunk, and Dan got a huge tattoo on his chest that was supposed to say “free man”…but instead got “Fremont”. They meet girls, Vicki (Kelly Preston) and Jenna (Rita Wilson), and drunken Dan marries Vicki, but they got it annulled the next day. Dan never remarried and hasn’t seen Vicki since.
Dan gets a message at work that Vicki is in town, and wants to meet for lunch. She tells him that she is going to jail for 2 weeks, to serve time for chaining herself to a fence to stop construction near her home in Vermont. She tells him that she is leaving her 2 kids, Zach (Connor Rayburn) and Emily (Ella Bleu Travolta) with her best friend Jenna during that time, and tells the kids she is going on vacation to a spa. The kids run in and yell, “Daddy!” before Vicki had the chance to tell Dan that he is the father of her twins. She wants them to spend the day together so the kids can get to know their father, and they go to a carnival.
When dropping the kids at Jenna’s house, Dan accidentally slams her fingers in the trunk, and when opening the trunk to free her, hits her in the head with the trunk. At the hospital, Jenna is in a hospital bed, with a neck brace and splints on all her fingers. She cannot take care of the kids now, so Dan offers to help out. In desperation, Vicki agrees. She leaves for jail, and a bewildered Dan seeks Charlie’s help for the next two weeks. Dan lives in an adults-only condo complex, so Dan and kids move in Charlie’s bachelor condo. Dan hires a safety team to childproof the house (Luis Guzman and Dax Shepherd).
The kids wake the next morning, dressed in their Pioneer Scouts camping outfits, and tell the guys that they have to take them camping for 3 days, and earn merit badges. Reluctantly, they go. Camp director Barry (Matt Dillon) hassles Dan and Charlie and calls them “grandparents”. They play a game of Ultimate Frisbee, and Barry calls for “Prison Rules” which means full tackle, anything goes, tackle Frisbee. Dan and Charlie get beat up. At skeet shooting, Dan misses and shoots the head off the statue of the original Pioneer Scout, and later burns it down in another accident. The 3-day camping trip ahs been reduced to 9 hours, and they go home.
While brushing their teeth, Zach and Emily knock down a shelf in the bathroom, where Charlie and Dan keep their daily prescriptions. They spill all the pills, and the kids quickly put them back, but just guess who get what pills.
As part of their big business deal with the Japanese businessmen, Dan must go play gold with them. After taking some of the wrong prescriptions, Dan has depth perception problems, and makes of mess of the golf game, hitting 2 of the players in the groin with a tee shot.
Dan has trouble connecting with his kids, so Charlie takes them to a live puppet show, put on by Jimmy Lunchbox (Bernie Mac). Charlie gets Jimmy to help Dan. Dan suits up in a human-puppet suit, and while he is having a fairy princess tea with Emily, Jimmy and Charlie are upstairs, watching on a laptop, and making the “suit” that Dan is wearing dance and have fun with Emily. It is a big success, and Emily hugs Dan. While unattended, Zach is playing a video game, and a video conference call comes on the TV, interrupting him. It is the Japanese, and they have questions. Zach types in answers, and says that their competitor is “poop”. They like his aggressive style, and they seal the deal, not know it was just a kid they were talking to.
Dan starts to do things that Zach has put on his list of things to do with Dad: learn to ride a bike, which they do in Central Park, and go to a baseball game. They go to a practice with the Mets, get autographs and stay for the game. While on the jumbotron, the text at the bottom reads, “Mets Grandparents Day” and Charlie and Dan wince.
Charlie and Dan’s partner, Ralph (Seth Green) is sent to Japan, to work directly. He goes AWOL, staying in Karaoke bars and never turns up. The Japanese are furious, and say the deal is off unless both Dan and Charlie move to Japan. Charlie is ready, but Dan is torn at the prospect of leaving his kids and Vicki behind. Later, he goes to pick up Vicki, as her 2 weeks are up, and she tells him that they should give their relationship another try, and be a family. Dan sits the whole family down, and explains that as much as he doesn’t want to, he must move immediately to Japan. They are sad.
Dan and Charlie fly to Japan, and when it is time for the big business meeting, Charlie begins, and when it is Dan’s turn, her messes it all up, seeing pictures of his kids on his computer. He leaves the meeting. Charlie follows and Dan tells him that their partnership is over, and he is returning to New York.
Upon returning, Charlie learns that his old, old dog Lucky, died while he was in Japan. He has a funeral and afterwards, Dan talks to him, and they make up when Dan brings him a new puppy. They decide to hurry and try to catch his twins’ birthday party at the zoo in Vermont. The 3 guys drive up, but find that they are too late to get in the zoo, so they sneak in. They end up in the gorilla enclosure, and make a break for it. Charlie and Dan climb the wall, and land on the other side in the penguin pool. Ralph is stuck with the gorilla, who is cuddling him in her lap like a baby. Charlie and Dan get attacked and pecked by penguins, and finally get out of the pool. Dan finds out that the kids’ birthday party is on Adventure Island in the zoo, and to get to it, he borrows a jet pack from a “superhero” that Vicki hired for the party. Dan flies in, impressing Vicki and the kids, and then crashes in the lake. He tells them that he just rented a house nearby, so they can be close, and the kids can see him as often as they’d like.
1 year later: Dan, Vicki and kids, plus Charlie, his new wife and baby, are on a yacht, heading to Aruba. The Japanese business men have called back, and want family men to head up their deal, and so the guys are back working. The safety crew is there, childproofing the yacht, as the movie ends, with Charlie and Dan, and their families with them.