Nicholas "Oz" Oseransky (Matthew Perry) is a Quebec dentist from Chicago, trapped in a miserable marriage to the daughter of his late, ex-partner. He can't divorce her because her father embezzled from the dental practice, and it would leave him with nothing but debt. As the movie opens, Oz comes down to the kitchen, where his wife, Sophie, and her mother are having breakfast. Sophie tells Oz she needs $5000, but won't say what for. He leaves for work, taking some frustration out on the steering wheel on the way.
At the office, we are introduced to his assistant, Jill (Amanda Peet). Jill and Oz have lunch and she tells him that his wife isn't a good person. They joke about having her taken out of the picture, and Jill tells him to name a price. When Oz gets home, a new neighbor is moving in next door. Oz thinks that "Jimmy Jones" looks familiar, and after recognizing a tatoo on Jimmy's forearm, realizes that he's Jimmy "the Tulip" Tudeski (Bruce Willis). Tudeski is an infamous contract killer from Chicago who has been given amnesty after ratting out Lazlo Gogolak and other members of the Gogolak gang.
After an uncomfortable encounter, Oz runs home in a panic and tells Sophie they're moving. But Sophie insists that Oz return to Chicago to collect a finder's fee by giving up Jimmy's wherabouts to Lazlo's son, Janni (Kevin Pollak). Upon arriving in Chicago, Oz has lunch with an old friend and confesses that he has no intention of meeting with Janni. Unfortunately, when Oz returns to his hotel room, he is greeted by Frankie Figs (Michael Clarke Duncan) who turns out to be one of Janni's enforcers. Frankie roughs Oz up to get the truth out of him and then takes him to Janni.
At Janni's estate, Oz meets Cynthia (Natasha Henstridge), Jimmy's estranged wife. Janni instructs Frankie to accompany Oz back to Canada and keep an eye on Jimmy until Janni and his men arrive to take him out. Back at the hotel, Oz is freaking out when there's a knock at the door. It's Cynthia. She asks if Oz really knows where to find Jimmy, explaining that Janni and Jimmy both want her (as well as each other) dead in order to collect on $10 million dollars being held in trust in their names. Oz woos Cynthia and the two share a bottle of scotch and end up sleeping together. Then Oz vows to find a way to protect Cynthia from Jimmy.
Back in Canada, Frankie and a very nervous Oz enter another hotel room and see Jimmy waiting inside. It turns out that Frankie and Jimmy are old friends and together, are planning to take out Janni and his men (and Cynthia) when they come to do the hit on him. Oz returns to work and tells Jill about the events in Chicago. Jill is extremely excited when she finds out Jimmy the Tulip is Oz's neighbor and makes Oz take her to meet him. Jill tells Jimmy that he's her hero and the reason she wants to be a contract killer. We find out that Sophie tried to use the $5000 to have Jill kill Oz, but Jill ended up liking Oz after getting to know him and refused the contract.
As Jimmy and Jill discuss the upcoming ambush, Oz gets angry and says that he won't stand by and let them kill Cynthia. Meanwhile, Sophie is in bed (literally) with a new hitter she has secured with her $5000. Oz is back at home and tries to call and warn Cynthia about Jimmy's plan, but she is already on the way with Janni. When they arrive at Oz's house, Oz gets a chance to warn Cynthia, while she warns him that Janni will kill him after killing Jimmy. The two watch from Oz's garage as Janni's gang walks into the trap.
Down the street, Sophie and her hitter are in his car. She is discussing the payment and promises that her mother will pay the remainder of his fee after the job is done. The hitter (played by Harland Williams) is distracted when he recognizes Janni Gogolak heading toward Jimmy's house. When Sophie tells him whose house it is, he realizes what's about to happen, pulls out a gun and exits the car, leaving Sophie bewildered.
When Janni enters the house, he is distracted by a naked Jill, and she, Jimmy and Frankie are easily able to kill Janni and his men. Sophie's hitter is in the yard when he hears Oz's car peel out behind him. Oz and Cynthia are making a break for it. This draws Jimmy's attention to the window. He has Jill get the mystery hitter's attention and then shoots him. Sophie sees this and runs, screaming, down the street. When they get the body inside, they realize he is an undercover cop.
As they start to get a plan together to dispose of the bodies, Oz calls. While they are on the phone, Jill reveals that Oz slept with Cynthia in Chicago -- which she thinks is great -- but it infuriates Jimmy. Oz makes a deal with them that will benefit everyone. He will alter the cop's teeth to exactly match Jimmy's. They then put the cop's body and Janni's into Oz's car and set it on fire, rendering the bodies unidentifiable, except by dental records. Jimmy is still really mad, but luckily, the plan works. The investigators think that Janni and Jimmy are both dead. This allows Cynthia to collect the $10 million, which as a part of the deal, she has agreed to transfer to Jimmy in exchange for her and Oz's lives. The investigators also find the undercover cop's car when they go to check out Jimmy's house. Inside the car is a tape recorder, the contents of which are more than enough to put Sophie and her mother in prison.
The next day, the key players meet up in the city. While Cynthia and Jill go to the bank to transfer the money, Oz heads to an art museum with Jimmy, where they will be around people and Oz thinks he'll be safe. Jimmy has other plans though and takes Oz (along with Frankie Figs) out on a private yacht. Back at the bank, Jill asks Cynthia if she's tempted to take the money and run. Cynthia decides that she loves Oz and can't leave him to be killed by Jimmy. Jill tells Cynthia that Jimmy wants her and Oz to have $1 million as a wedding gift, and the two women make the transfers.
On the boat, Jimmy gets the call that the money has been received. He pulls out a gun and points it at Oz. Frankie grins knowingly before Jimmy turns and shoots him instead. Oz is floored, but happy to be alive. He tells Jimmy that it's obvious he's in love with "her." Jimmy acts confused, thinking Oz is referring to Cynthia. In the next shot, Jill runs on the boat and jumps into Jimmy's arms. Oz leaves to meet Cynthia. As soon as he sees her, he asks her to marry him. She looks disappointed and asks if Jimmy told him (about the money.) Oz doesn't know what she's talking about, and so she happily agrees to marry him. In the closing scene, the two newlyweds are on a hotel balcony overlooking Niagara falls, dancing to a bluesy version of Ira Gershwin's "They All Laughed" as the credits roll.