The film begins with Ronny (Vince Vaughn), his girlfriend Beth (Jennifer Connelly), his best friend Nick (Kevin James), and Nick’s wife Geneva (Winona Ryder) all at dinner. They’re sharing stories when Beth asks how long it takes to really know someone. Ronny states that you can never really know someone; Nick disagrees.
At a car show, Ronny is able to get him and Nick a meeting with General Motors in a few days. They celebrate with dinner with their ladies. While Nick and Geneva are dancing, Ronny tells Beth that he really looks up to Nick, who pulls him onto the dance floor. When they switch partners, Geneva tells Ronny that they love Beth and asks Ronny when he’s going to propose to her. That evening, Ronny asks Beth if there’s a clock ticking; she says no, but if things progress, she won’t be unhappy.
In Detroit for their meeting with GM, Ronny tells Nick that he’s worked out a deal to get Beth a $20,000 ring for half price if Nick can help by fixing the seller’s car. Nick agrees. In their meeting, they pitch an electric car with the build of a muscle car. GM agrees to give them $400K for a prototype, and leaves Susan Warner (Queen Latifah) as their supervisor. She’s extremely enthusiastic about working with them, and talks about having serious “lady- wood”. Nick worries that he’s gonna fail; Ronny reassures him.
Later, he mentions this to Beth, who’s chef, who asks how Ronny is doing, making sure he’s not feeling overwhelmed. (Ronny had a very serious gambling problem two years prior.) He promises that he’s fine then heads to a botanical garden to arrange the proposal. While he’s there, he catches a glimpse of Geneva, whom he follows. He walks through poisonous plants and is caught by the manager just as he sees Geneva kiss a young man (Channing Tatum). He’s read all of the horrible side effects of the plants, (painful urination, hallcinations, etc.) and kicked out. He later lies to Beth about how he got the rash, and when he re-enacts it, he drops a stack of money, which Beth sees. He lies again saying that he’s paying his parts suppliers, which she doesn’t appear to believe.
He goes to work to tell Nick, but doesn’t when Nick yells at him. He calls his sister for advice, but she assumes he’s talking about her husband, so that fails too. Nick later apologizes, and before Ronny can say anything, Beth and Geneva come to take them out for a break. They go to a BlackHawks hockey game. Geneva goes for beer; Ronny follows and confronts her. She tries to explain, then accuses Nick of getting “massages” every Thursday. She promises that the affair is over, and she’ll tell Nick once the car is finished.
At the office later, Nick inquires about why Ronny’s dragging his feet with the proposal. Ronny tries to probe about possibly getting a massage; Nick feigns ignorance. Ronny later follows him to the massage parlor. He then calls Geneva to apologize, but hears her with the young man, Zip. She tells him to meet her at a diner, where she tells Ronny that she’ll deny the affair and tell Nick that Ronny has been hitting on her, and revealing a fling they had back in college. She leaves telling Ronny to stay out of her marriage.
At home the next morning, Beth gets a call, but the caller hangs up. He calls back, and gets the number to a corporate office that is opening a restaurant in Vegas. He later gets a call from Susan, warning him that GM has another competitor working on a similar project. Afterwards, he follows Geneva to Zip’s and photographs them together. Kids start skateboarding nearby, so he can’t leave, making him late for Beth’s parents’ 40th anniversary party. When Geneva leaves, he’s caught by Zip, and they get into a fight. He makes his way out by spraying hairspray in front of a lighter. He gets to his car, which Zip attacks with a baseball bat. Zip asks who he is, so he explains. He calls Zip a nothing, which hurts Zip’s feelings. Zip walks away, beginning to cry. Ronny cleans himself up then heads to the party. He apologizes and makes a toast about honesty. Beth takes him aside and begs him to be honest with her, so he asks about the Vegas restaurant. She explains that she was offered a job, but declined because she doesn’t trust him in Vegas. He leaves and spends the night at the office.
He goes back to Zip’s to get his camera. Zip answers the door with a gun, and insists that Ronny come in. (He googled that if someone enters your home, you can shoot them in self-defense.) He then cries about Ronny killing his fish and demands $10,000. Ronny offers $1000 and an apology. Zip gives him back his camera. Nick arrives after Ronny leaves, having followed Ronny.
Ronny gets to his office to find Beth, Nick, his sister, Geneva, and some others holding an intervention for him. Zip shows up seconds later, claiming to be his bookie. Ronny lashes out at him and hits him. When Zip leaves, Ronny explains that he’s not gambling, he’s been acting weird because he found out a week ago that Geneva was having an affair with Zip and didn’t know how to tell Nick. Surprisingly, Geneva admits the affair. Ronny also confesses to the college fling. Nick is outraged and leaves.
Ronny comes home later after going for a walk. He tells Beth he brought her something to eat and they apologize for not trusting each other. She opens the bag of food and finds an engagement ring inside, and accepts his proposal.
At the meeting the next morning, Nick tells Ronny he moved out, then hits Ronny twice for not telling him the truth. He makes Ronny promise to be honest even if it hurts. They make up and head into the meeting. Nick powers on the car and it roars like a muscle car should, impressing GM and winning a contract with them.
At a BlackHawks game some time later, Nick finally get a chance to try a challenge he always claimed he could do. He fails twice, but on his third attempt, Ronny yells some words of encouragement, and he gets the puck into a net, winning a chance to go to an all-star game. Ronny runs onto the ice to celebrate with him before they are both escorted off.
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