The movie starts with everyone preparing for a wedding. The bride, Libby (Thandie Newton) is making finishing touches to the cake. In walks Gordon (Dylan Moran) who comments in a sexually suggestive manner that she's good enough to eat. Libby reminds him they're cousins. He jokingly remarks it never stopped the royal family. The camera shows a shot of Libby, where we see she's pregnant. She asks Gordon how Dennis (the groom) is doing upstairs. Gordon remarks that he's thrilled.
Cut to a shot of Dennis (Simon Pegg), who's having a panic attack. Gordon checks on him, and decides to give him a minute. Eventually Gordon knocks on the door again, repeatedly telling him it's time. Dennis continues to panic, and takes off through an open window. The entire congregation watches him run and Libby yells at him.
Five years later, we see Dennis as a security guard running. He's chasing a drag queen who's stolen merchandise from a clothing store. The thief is having fun with the fact Dennis isn't very fast. However, he runs into someone and falls. This gives Dennis the opportunity to wrestle away a pair of panties he stole. Dennis returns with the merchandise, but the clerk asks where's the bra. Dennis turns around to see the same thief showing off the bra and remarks "C'mon fat boy, run!" Dennis replies that he's not fat, he's just unfit. The chase begins again.
The next scene is of Dennis walking home from work. He lives in a tiny apartment below his landlord, Mr. Ghoshdashtidar (usually referred to as Mr. G, played by Harish Patel) and his incredibly beautiful but demanding daughter Maya (India de Beaufort). Dennis has a lot of back rent to pay, and the two aren't shy to let him know. Eventually they let him in anyway.
The next scene is of Dennis and Libby's child, Jake (who lives with Libby). He is preparing to see a Lord of The Rings musical with his dad. Back at Dennis' apartment, he dresses and flirts in the mirror (taking it a bit too far). At Libby's, we see her interact with her new boyfriend, Whit (Hank Azaria). Dennis finally arrives, and we see him interact with his son (playfully calling him "snot face"). A not too thrilled Dennis is then introduced to Whit. Libby asks him if he got tickets to the show already. Dennis suggests that Libby "credit him with some intelligence".
The next shot is of Dennis yelling in the middle of a crowd, looking for scalpers. He finds one, and purchases tickets, but it turns out to be an undercover cop. Dennis is taken in and Libby and Whit pick up Jake at the police station. Libby and Jake take a taxi home. Whit takes a walk with Dennis and we get a little background on the two. Whit is an American businessman from Chicago who met Libby at her bakery. Dennis tries to make his own job sound sophisticated (a security guard at a women's clothing store). Whit also lets him know his hobbies, including long distance running. It turns out he is running the Nike River Run--a marathon race. Dennis asks why he would do that. Whit lets him know it's for charity. As far as Libby and Jake, Whit tries to alleviate the awkwardness by being upfront about his feelings for Libby.
Dennis rides the public bus back home, remembering the happy times he had with Libby, but ultimately ending with the reminder he left her at the altar. Back on the bus, Dennis waves hello to a sleepy child on his father's shoulder, which the child replies by giving him the finger.
Dennis wakes up the next morning and goes to Vincent's (who owns a Piano store, which appears to be a front for a gambling center). In the back room he finds Gordon playing poker with a few others. After the game, He and Dennis chat about Dennis' night. Gordon gives him the movie tickets, which were only good for yesterday. Dennis unloads about his feelings for Libby. Gordon reminds him of the day he left her, and how he should think about stopping his pursuit. He will however help in any way he can. Dennis asks for some money to borrow to pay his rent, to which Gordon laughs and thanks Dennis, saying he needed that laugh.
Dennis then spends the day with Jake, hiding up in trees and making weird noises to scare joggers below. Jake talks about a girl he likes in his class named Emily. He says, "I really think we can make a life together". Dennis stops him saying he sounds like Whit. Jakes says that he picked it up from a conversation Whit had with Libby. Dennis calls Whit a shithead, which he apologizes for right away. Jake then lets him know that Whit got him front row tickets for the show. Dennis calls Whit a shithead again.
Dennis then goes to The Gherkin building where Whit works. He has a hard time getting through the receptionist who mistakes him as a deliveryman. He sees Whit and asks him if he wants to get a pint. Whit says he can't, since he has a spinning class he needs to get to, in order to continue his training. He invites Dennis to go with him. At the class, it's obvious Dennis is not nearly as in shape as everyone else.
In the locker room, a naked Whit applies powder to his crotch asking Dennis if he wants any. Dennis declines. He asks Whit if he wants to skip the show with Jake, since he might not want to be stuck in a theater for three hours. Whit sees where this is going and reminds Dennis that he also wants to be a growing part of Jake's life. He asks Dennis if he sees his point. Dennis says yes, obviously referring not just to his words, but Whit's exposed penis. On the way out of the gym, Whit warns Dennis about walking down the stairs after the grueling class. Dennis ignores the advice and ends up tumbling down.
Dennis decides to stop by the bakery to see Libby. He has to get an order number to get a chance to talk with her since she's so busy. He uses the opportunity to talk to her about what she sees in Whit. The store is busy and he has to compete with pushy customers who hear and comment on their personal business. To prove that he has changed, he says that he'll run the Nike Run Marathon.
Back at the poker room in Vincent's, the players laugh at the visiting Dennis, who's told them about his plans. Gordon, however, believes in Dennis and says he can do it. Dennis gives a speech about how he's tire of being a coward in life and that it all ends at 6am.
The next morning (at 8am), Dennis wakes up and begins his training. He jogs about 1 block in tiny shorts before getting side stitches. The shorts were a bad idea since he ends up with a rash he can't stop scratching. He meets up with Libby to walk Jake to school. Whit ends up tagging along. He lets Jake know that he has an extra ticket, and they should invite Emily. He also comments on the news that Dennis running the marathon. Whit says that even though the marathon is just three weeks away, he could still get in if he runs for a charity.
During work, Dennis calls several charities to see if he can run for them. However, most of them want him to wear a costume while doing so. He says he'll get back to them. Dennis' boss tells him to refrain from scratching his scrotal zone in front of the customers. He ends up using a mannequin's hand as a scratcher, which creates a humorous view for the customers.
Back at home, Mr. G. and Dennis have a cup of tea. Mr. G. comments that running this marathon could be a great start towards being a more courageous man. He also reflects about his deceased wife. He talks about the joy of spending everyday with his best friend, "But most of all, I remember all the fucking", which makes Dennis very uncomfortable.
Dennis spends the days with Jake at the park, playing with a remote control boat. Libby and Whit come by to spend time with Jake as well. Libby talks to Dennis about how running in the marathon won't really change anything. Whit outdoes Dennis' boat by steering a huge remote control pirate boat, complete with little cannons. Jake asks Whit if he can steer it. Whit says no, but he can watch. Another boat bumps into Whit's. He has a mini-freak out and we can see the cracks in his character.
Back at Vincent's, Gordon asks Dennis if he really plans on running the marathon. He says yes, and Gordon says "thank god". It turns out Gordon is betting heavy on Dennis and hopes to win big, since the odds are against him. At 6am the next morning Gordon wakes him up by banging pots and pans. Now that he has an investment in Dennis, Gordon has become his trainer. Mr. G. is the assistant trainer, who hits him with a spatula every now and then.
Back at home Dennis treats an enormous blister. Gordon hands him a needle, but Dennis is too afraid to do it. Gordon says he'll do it on the count of three (but does at the one count). The needle gets stuck in the foot, and when Gordon finally gets it out, a huge gush of liquid explodes in Gordon's face. Dennis says he's sorry and "that's never happened before". Gordon claims that is the second most disgusting fluid that's gotten in his eye.
Mr. G., Gordon and Dennis go the Nike offices to present their fake charity. Gordon claims Mr. G. suffers from a rare disease, which involves paralysis, outbursts, and chronic flatulence (which Mr. G. shows examples of, except the flatulence). The Nike officials say they'll approve, but need their charity's registration number. Obviously the three men don't have one, and their ruse is finished (just as Mr. G. gathers up enough gas to pass).
The three walk dejected, pushing the empty wheelchair, when they lose control of it. The chair goes down a flight of stairs and takes out a man. They help him, but he complains that he won't be able to run the marathon for his charity now. Dennis talks him into taking his place. The charity: Erectile Dysfunction (as shown on his shirt later).
Dennis continues his training in a short montage, which shows some funny shots but also some improvements. After taking a shower back at his place, Maya is waiting for him. She says she has a crazy bet of her own. She will forgive the back payments if Dennis completes the marathon. If he doesn't, he moves out and never comes back. Dennis is reluctant to agree, so Maya takes off his towel and takes a Polaroid. Dennis agrees to the bet. Later Mr. G. comes by and gives him a pair of new running shoes as a gift.
Dennis and Gordon go to Libby's birthday party, held at Whit's apartment. Libby shows Gordon to the restroom and talk on their way there. Gordon says Whit looks like he came out of a catalog, but warns how he ordered a pair of Italian shoes out of a catalog that "hurt like fuck". After seeing the impressive bathroom, Gordon asks would be weird if he took a bath. She doesn't answer, and he assumes it would be too weird.
Whit shows Dennis his view, and points out St. Patrick's cathedral, the finishing point for the marathon. He becomes frank with Dennis and says that running a marathon is tough, and that it takes discipline and perseverance. He feels Dennis doesn't have those traits. He says that eventually every runner "hits the wall". After that, it all depends on whether the runner is a quitter. He says that's the difference between getting a girl like Libby and keeping a girl like Libby. Dennis later bumps into Libby and they have a chat. We can see there is still chemistry between the two. He says that rather than winning her back, he'd rather at least win her respect. He gives her a gift to open later.
Later at the party, Whit gives a speech to everyone with Libby at his side. He finishes the speech by asking her to marry him. She thinks about it for a few seconds, but says yes. The crowd cheers, and Dennis is devastated and leaves. Gordon comes down in a robe and a towel over his head (we assume he changed his mind, and took the bath) asking what did he miss.
Dennis gets drunk and goes back home, looking at pictures of him and Libby. Libby looks at her gift: a photo of them back at a Halloween party. The next day Libby and Dennis meet to have breakfast. She says she is concerned since Jake isn't taking the engagement news well. However, Dennis is too angry at her about the engagement to begin with, eating an unhealthy breakfast "with extra breakfast on the side".
Dennis goes to Vincent's and says he is out of the marathon. Gordon is upset and the rest of the gamblers seem happy (they're betting against him). Gordon takes Dennis outside and tries to talk with him. They have an argument, which eventually turns into a short fight. Dennis goes back home, only to find his things waiting for him on the curb. Maya is throwing him out since she just heard he's not running the marathon. Dennis goes to Mr. G and tries to talk some sense into him. However, he is too disappointed in Dennis. He tells Dennis how he always has some excuse, but really it's all his fault.
Dennis goes to Libby's and congratulates Whit, only to learn that Jake has just run away. He goes to Gordon and asks for help. Gordon tells him to calm down and go somewhere he would go. Dennis heads to the park and finds a dejected Jake. He calls Libby to tell her Jake is fine before having a chat with him. It turns out Emily has just broken up with Jake. Dennis tells him these things are going to happen, but he can't just run away from his problems. He then realizes that's what he's been doing all along.
Dennis goes to Gordon's. Gordon walks out on the balcony and asks if he found Jake. Dennis says yes, to which Gordon replies that he can't stay with him (still mad about their fight). Dennis however informs him that he is back in it. An elated Gordon says, "Come on up then". The camera reveals that Gordon is naked from the waist down.
Dennis gets ready to sleep on the couch, while Gordon gives him a huge plate of pasta (a high carb dinner) and KY Jelly for his nipples. Dennis dismisses it (but later uses the jelly) and goes to bed, reminding Gordon they can't leave any later than 8 am.
The next morning, the two wake up at exactly 8 am. They panic and end up catching a taxi. Gordon trades in his watch to pay the fare, and Dennis just makes it to the race. He bumps into Whit and even the drag queen from the start of the film. Whit and Dennis start the race next to each other and talk during the first few minutes. However, Whit uses the opportunity to tease him and says he'll have to get over Libby, since he wants her to move to Chicago with him.
The two start getting really competitive and start running faster (despite it being the first mile of a 26.2 mile race), sprinting pass the serious runners. The broadcasters take note of this strange choice. Gordon (with the other gamblers) yells at the TV and tells Dennis to slow down. Dennis keeps up with Whit a little too well, so he decides to trip Dennis, taking them both out. We can tell that Dennis has just sprained his ankle. Whit gets back up and continues. The other gamblers celebrate, Gordon drops his head in disappointment, and Mr. G. goes to the race to help him, and Libby and Jake go to the hospital to wait for Dennis.
The other gamblers confront Gordon about the money owed, and Gordon admits he doesn't have it. Back at the hospital, Libby and Jake arrive and ask where the injured runner is, only to find out it's Whit. He claims Dennis tripped him and that he could have a serious injury. Jake plays with the bed control, so Whit asks if he could control the bed. Jake replies "No, but you can watch me". Back at Vincent's, Gordon is about to have his hand crushed in a piano lid. Just then, Mr. G comes to the rescue. However, we find out, that he just promised Vincent he would bring Gordon back to get beaten.
The two coaches find Dennis and we see that he's continuing the race, despite the ankle. Vincent and the gamblers learn the news and go to find Dennis themselves. The media and the fans start to be captivated by the mysterious runner. Back at the hospital, we learn that Whit doesn't have any real injury and that he just probably "hit the wall". Whit doesn't believe it and yells at Jake for continuing to play with the bed calling him "a little shit".
News reporters try to interview Dennis, but his speech is too laced with expletives to broadcast. Vincent and his crew can't interfere in the race, but they do follow and boo him, while Gordon and Mr. G cheer him on. Whit, Libby and Jake go home. Whit apologizes for the outburst earlier, explaining his emotions got the best of him. They also turn on the TV, only to see the media around Dennis.
With 9 miles to go, Dennis finally hits the wall. During this moment he imagines an actual wall in front of him, with no one else around. One brick is pulled out, and Dennis looks through. On the other side is an image of himself encouraging him forward. Dennis then starts to repeatedly throw his body through the wall, harder and harder each time. During this build up, we see the images of Dennis' panic attack from the start of the film, and all the events leading up to this moment. He then opens his eyes, and per serves. The reporters also show a replay of the trip from the start of the race, which shows the guilty Whit. Whit apologizes to Jake and Libby, but leaks his plan to move to Chicago. Jake and Libby walk out on him, with Jake saying "What a shithead".
Gordon keeps encouraging Dennis (now with a homemade splint) saying, "You are so close to me going on holiday". He arrives with less than 100 feet to go, when Dennis collapses in fatigue. It seems he can't go on. Just then, Libby and Jake arrive at the finish line. The sight gives him the motivation to get up and sprint the remaining distance to the end. He collapses into her arms. The crowd celebrates.
The next scene is of a more fit Dennis (we learn he now runs every day) walking towards Libby's. He goes to pick up Jake to spend the day with him. He wants to ask Libby something, but changes his mind. However, just moments after Libby closes the door, Dennis knocks again. She answers and he asks if she would want to have dinner with him. She thinks about it for a few seconds, but ultimately says yes. Dennis and Jake then go to kick a soccer ball around, when they're met by Mr. G, who just got back from a spin class. The three end up kicking the ball around together.
We then see Gordon on a nice boat, giving a summary of how he motivated Dennis throughout the race. He's talking to two women who seem very impressed. Gordon tells them, he has a bottle of champagne if they want to hear the rest of the story. The two say "sure". Gordon replies, "Come on up then". The camera shows he's once again naked from the waist down.
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