Ronny eventually tells Nick the entire truth and Nick and Geneva get a divorce. Ronny and Nick's friendship survives. Nick's engine succeeds, Ronny gets engaged and married to Beth, and Nick wins a trip to the NHL all-star game.
Ronny Valentine (Vince Vaughn) and Nick Brannen (Kevin James) are best friends and business partners. Ronny is a genius marketer and Nick is an engineering savant, though Nick also suffers from low self-esteem and has experienced several recent failures.
Things seem to be going their way when Nick designs a powerful electric engine and Ronny gets Dodge to invest in the development of the engine with an option to purchase it if successful. Further, Ronny is truly inspired by the seemingly happy marriage between Nick and Geneva (Winona Ryder) and is considering proposing to his own girlfriend, Beth (Jennifer Connelly). The foursome constantly double date, often going to hockey games where Nick always brags he could win a contest that requires the contestant to make a goal from center ice.
When Ronny visits some local gardens to plan a proposal for Beth, he accidentally catches Geneva making out with a young man named Zip (Channing Tatum). When he goes to see Nick and tell him of Geneva's infidelity, he finds Nick in a self-loathing rage as the engine is not developing as Nick had hoped. Ronny stays silent after seeing how upset Nick is and after talking to several other people (and in the process, inadvertently exposing an affair his brother-in-law was having on his sister).
Ronny does confront Geneva over her infidelity. She says she is ashamed, but says that Nick has been ignoring her for years to focus on his work and that he also takes weekly visits to a massage parlor where he gets "happy endings" (which Ronny essentially discovers is true). Nonetheless, Geneva promises to stay away from Zip and toconfess her affair after the Dodge presentation. Later, however, Ronny again catches her with Zip and threatens to tell Nick of the affair immediately. At that point, Geneva says she will tell Nick that Ronny is lying, expose the fact that she and Ronny had had sex prior to Geneva dating and marrying Nick (a fact that Beth is also unaware ) and falsely claim that Ronny has been trying to seduce her ever since. Further, the engine is still not working and Dodge liaison Susan Warner (Queen Latifah) tells them that Dodge has demanded they move the presentation up immediately or Dodge will pull out of the deal.
Ronny begins becoming paranoid that Beth is cheating on him and keeping Geneva's secret is driving him crazy. He spies on Zip and Geneva having sex and photographs them in the act so that he can confront Nick with undeniable proof. After Geneva leaves, Zip (who is drug-addled) catches Ronny peeping through the window and they get into a huge fight. Ronny shows up both late and badly beaten to Beth's parent's anniversary where he makes an ass of himself. Ronny returns the next day to apologize to Zip for their fight, not knowing that Nick has followed him and thinks that Ronny has relapsed (he used to be a gambling addict) and that Zip is his bookie.
Accordingly, Nick, Geneva and Beth hold an intervention for Ronny. Zip shows up because Nick agreed to pay him to support Ronny. Eventually, Ronny tells Nick that Zip and Geneva are having an affair and also, that he and Geneva slept together Nick began dating her. Geneva doesn't deny it and Nick leaves in fury. Beth consoles Ronny, they agree to always be honest with each other. Ronny then successfully proposes to Beth.
The day of the presentation arrives and is also the first time Nick and Ronny have seen each other since the intervention. Ronny lets Nick punch him several times before they go in the meeting with Dodge, though Nick says that he still loves Ronny like a brother and ultimately thanks him for being truthful. Nick confidently starts the engine, which works great, and Susan signs the duo to a multi-million dollar production deal.
We skip ahead to see that Ronny and Beth are happily married, though Nick and Geneva have divorced. Nick won a chance to shoot a goal from center ice and misses his first two shots. Ronny yells out some motivation before Nick's final shot and Nick confidently scores, thus winning a trip for two to the NHL All-Star game. Ronny jumps on to the ice where the two happily embrace.