The movie opens with Jimmy (Bruce Willis) and Paul (Tracy Morgan) in the interrogation room, working on a suspected drug dealer. Paul convinces Jimmy to let him try the interrogation this time and the crazy scene with all the movie quotes from the trailer ensues. They get the guy to rat out his supplier, so the next scene is at the guy's cell phone store, where Paul works as a sales person on the street with a cell phone costume on. He's casing the place, waiting for the supplier to show up so he can bust him, but when he does, the informant accidentally tips off the supplier who shoots him and splits out the back door. Paul chases him in the cell phone costume and eventually beats up a kid and steals his bike to chase the supplier. This is taped and put on YouTube, and when they don't catch the guy both cops are suspended without pay.
Next, we see Jimmy and his ex-wife, daughter, and the ex-wife's new husband out at dinner. The daughter, Ava (Michelle Trachtenburg) wants an elaborate wedding (we see a bill for $48,000). Jimmy can't afford to pay for that, least of all now that he's suspended without pay. The new husband Roy (Jason Lee) offers to pay for it, but it's more to humiliate Jimmy than to help Ava. Jimmy can't let that happen, so he decides to sell his one of a kind baseball card, worth up to $80,000. When he arrives at the card shop to sell it, the shop is robbed by Dave (Seann William Scott) and an accomplice, who take money and memorabilia, including Jimmy's card. Jimmy sees Dave's distinctive tattoo and he and Paul figure out which shop he got it at, pressuring the tattoo artist and get a tip on the house he's going to rob next. They stake out the house before Dave gets there and once he arrives, they capture him and find out who's buying all the stuff from Dave.
The buyer is drug overlord Po' Boy, who's brother was the supplier in the cell phone store. Jimmy and Paul go to see Po' Boy, and he agrees to give them the card back if they find his new car (which was stolen en route to him). The car was sold to a Russian lawyer, and they track him down. Once they get the car (and get Po' Boy's brother killed in the resulting chase), they decide to figure out why it's so important to Po'Boy, and they find Gabriella (Ana de la Reguera) in the trunk. She doesn't speak any English, so they take her to a diner to a guy they know who speaks Spanish, and he tells them she is the mistress of a Mexican drug lord. They know she's in trouble so they take her to a hotel to keep her safe, and just then Jimmy gets a call from some other cops who tell him Paul is a dirty cop (because his gun was used in a couple of gang killings after Dave stole it during the card shop robbery and sold it to Po' Boy). He rebukes them and goes back to the hotel, where he finds Paul asleep and Gabriella escaped through the window (saying she doesn't want to cause them any trouble by hanging around with them). She has left her crucifix on the table, which turns out to be bank account information for lots of accounts linked to drugs, which Po' Boy was attempting to steal.
So now, knowing that the entire Po' Boy operation will be at the funeral for Po' Boy's brother for about an hour, they bail Dave out of jail so he can help them steal the card back. He promptly slips off the roof and is knocked out (setting up an in-credits gag where he wakes up in the morgue). Jimmy gets himself to the second floor and is still there when the organization returns to the house after capturing Gabriella off-screen. The other two cops who accused Paul show up (they had been working the case as well), and one is shot, prompting the other to call for backup to the house. This is when Paul is roused from his depression, figuring out that his wife, Debbie (Rashida Jones) isn't cheating on him, after all (a side-story running throughout). Paul and Jimmy proceed to clear the house of gangsters, leaving them with Po' Boy -- who has Gabriella hostage. They both shoot and both hit him, killing him instantly.
After securing Gabriella with the paramedics, Jimmy returns to the house to find his baseball card, only to discover that it was in Po' Boy's front pocket, and Paul had destroyed it with his shot.
The movie closes at the wedding, which Roy paid for. Jimmy's ex-wife Pam (Francie Swift) asks Jimmy to give away the bride with Roy (since Roy paid for the wedding, and all). He agrees, mostly because she threatens him. At the fateful moment, however, we see Paul (who is sitting behind Roy) put his gun in Roy's back, threatening to shoot if he stands to give away the bride -- allowing Jimmy the privilege all alone.