Ambitious high-flyer Nat (Rose Byrne) and struggling writer Josh (Rafe Spall) fall in love at first sight at a party. After seven months together they decide to marry. The film highlights their struggles during their first year of marriage, switching back and forth from flashbacks of the year's action to a marriage-guidance counselor's office. Their wedding goes as planned despite many friends' comments that the marriage will not last, an embarrassing best-man's speech, and a coughing priest.
When Nat returns to work after the honeymoon, she's embarrassed when Josh calls her in the office - on speakerphone in front of her colleagues - to tell her she is sexy and that he misses her, causing her to abruptly hang up on him. Later, the two meet with their solicitor to discuss how to handle medical crises (last wishes). Nat becomes annoyed when Josh, knowing she would be late, admitted that he deliberately told her the wrong time, causing her to turn up early.
The couple throw a dinner party to use their wedding gifts. Some of their differences are highlighted when they talk about their honeymoon in Morocco: Nat didn't enjoy the leather museum; Josh remembers it as interesting. When the topic changes to Josh's former flame, Chloe (Anna Faris), Nat discovers that the two never officially broke up when Chloe departed to Africa for four years. In the kitchen Chloe apologizes to Nat for not realising she didn't know. The women talk about the constrictions of marriage. Nat's sister Naomi has issues with her own husband's annoying habits. Josh's best man Danny asks Chloe out but is rebuffed.
The following day, Nat and her work-colleagues make fun of their new client, Guy Harrap (Simon Baker), the new owner of a bleach company. They believe he will be a stereotypical American who thinks the British are "quaint". They do not realize that their client has been sitting right there in the same café. Before the meeting, one colleague steals Nat's wedding ring, believing that the account will have a better chance of success if she appears single. During the meeting, Guy deliberately fulfils their expectations of him: speaking in a brash American way, asking for high-fives and casual fist-bumps, asking Nat to repeat certain words he finds amusing and doing a crude Austin Powers impression. Then when they focus on business talk, he switches to his true self, embarrassing the women for their earlier stereotyping. As he and Nat exit the boardroom, she apologizes for their misjudgment of him, and he says they should get better acquainted for the sake of the account. Feeling the attraction between them, she struggles with telling him she's married, then ends up leaving without telling him.
Josh talks to Chloe about his book while she's working at a charity office. He invites her to dinner because Nat's going to a work party that night. Chloe declines, saying she's going out with her work-colleague Charlie, whom she's been dating.
The scene returns to the marriage-guidance counsellor's office as the two explain that the realities of marriage do not live up to the fairytale expectation they both had.
Unable to focus on his writing, Josh sits at home watching television while Nat's out jogging. At work, Nat receives a large bouquet of roses from Guy. The couple bicker over domestic issues; Josh leaving the toilet seat up, Nat's inability to sing the right words to popular songs and their different definitions of the rubbish bin being full.
Guy shows Nat around one of the factories he owns, where one of his longest-serving workers expresses approval of her as a potential wife for him. Guy explains that he basically grew up in the factory during his childhood summers. Nat comments that she's not the marrying type, still unable to tell Guy she's married.
Nat tries to discourage Josh from accompanying her to a work party, but he is determined, irritating her. At the party, he makes a fool of himself with embarrassing dancing and standing next to a poster he can joke about during the night. When he approaches Nat while she's talking with Guy, she still doesn't reveal that he is her husband and Guy attempts to shake him off, assuming he's an unwanted menace. Guy asks her to dinner and Nat declines. Incredibly annoyed at Josh for embarrassing her at the party, she heads home without him.
Meanwhile, Chloe and Charlie attend a boring dinner party, then leave early to adjourn to Charlie's apartment. As they kiss on the bed, Chloe's colleague Alexandra joins them and Chloe finds herself in an awkward threesome. Feeling too silly to continue, Chloe eventually leaves. The next morning she calls Josh to tell him about it, and he soon turns up at her apartment with coffee and her favourite sweets to cheer her up.
Chloe and Josh then go Christmas-shopping. Josh wants to get casserole dishes for Nat but Chloe laughs that this is not a present for a wife and she must help him; they end up at a lingerie shop with Josh uncomfortably trying to make conversation with the shop assistant amongst the shop's expensive contents. Chloe tries on a lingerie set, and asks Josh what he thinks of it. They end up kissing in the dressing room, although both are embarrassed about it afterwards. Josh ends up buying the lingerie.
When Nat meets with Guy at his hotel to discuss their business deal, she rebuffs his attempts to get her into his room. He mentions that he has booked a conference room down the hall, but when Nat enters she finds a romantic dinner complete with doves and a violinist. When Guy makes advances, she finally blurts out that she's married and can't leave her husband because it would destroy him, and finally storms out.
Guy chases after Nat and they bump into Chloe and Josh on the street. After some initial awkward exchanges, Josh suggests that Chloe and Guy get together and they agree on a double date.
Back to the present in the counsellor's office: Nat explains that they hit a low point around the Christmas period, commenting that her husband's family are weird - in particular his mother. Josh retaliates that Nat's family were not overly friendly towards him.
The scene shifts to a Christmas family reunion at Nat's parents', where a series of embarrassing incidents revolving around Josh occur. Josh unwittingly but clumsily offends Nat's grandmother during a game of charades, Nat's father makes him sleep on the upper deck of a bunk bed of a young female relative, and Nat's parents giving Josh a pair of books titled "How to be a Successful Writer" and "How to Stop Wasting Your Life". At the end of the visit, while leaving her parents' house, Nat confronts Naomi about why she stays with her husband as they clearly hate each other. Naomi says that they both "embrace the hatred" and that's what marriage is about. Even though she admits there could be something better out there for her, she ultimately loves her husband.
Nat and Josh have a conversation about his suggestion of Chloe dating Guy. The two talk about the prospects of both of them as romantic interests. The four meet for dinner, and spend the evening playing pool. Chloe and Guy seem to hit it off, happily competing against Nat and Josh. Nat becomes more frustrated with Josh's clumsy and patronising attempt to teach her how to play properly, as well as with her growing jealousy towards Chloe, who can play well. They leave the bar, and Nat asks Guy to talk about packaging details, intending to meet Josh back at their flat afterwards. Chloe and Josh depart together, while Nat and Guy go the other direction. After a moment, Nat passionately kisses Guy, resulting in the ripping of the underwear bought for her by Josh.
Meanwhile, Josh attempts to discourage Chloe's attraction to Guy, and she admits she is and has always been still in love with him, lamenting that Josh never stopped her from leaving and insisting that their current circumstances are impossible, that they cannot see each other anymore.
When Nat returns home, she and Josh talk about their relationship. After nine months they decide to get help instead of giving up on their marriage. This leads us back to the counsellor's office, who ultimately advises them to try to make it to the one-year marker.
The couple then put up with each other's quirks over the next few months, eventually making it to their anniversary. Nat brings out the same expensive lingerie for the special occasion, and struggles to do it up because of two broken hooks, remembering the circumstances in which they were broken - her with Guy. Josh meanwhile leaves the flat, telling Nat he's remembered he has to do something and that he will meet her at the restaurant. He races to Chloe's apartment, only to find that she is heading off in a cab with Guy, whom she embraces lovingly. Nat contemplates phoning Guy, but then decides to go to the restaurant, where her friends and family are there waiting to surprise the couple. After failing to contact Josh, Nat sits down. She discovers that their friends didn't think her marriage would last.
Josh makes it to the restaurant party, and tells Nat that he thinks she is the perfect wife, just not for him. He asks her for a divorce and she immediately and delightedly agrees. The couple rejoice at the situation, and immediately leave the party one after the other.
Meanwhile, Guy and Chloe are at the railway station waiting to go to Paris on a romantic trip. Josh finds them and professes his love for Chloe. When it's discovered that he split up with Nat, the two are shocked. Nat appears behind Josh, who awkwardly assumed he is the one she wants to speak to, but it turns out she was there for Guy. After a short exchange they happily discuss how perfect Guy and Chloe are for them. In the end, Chloe and Guy mutually break up. Nat ends up kissing Guy and Chloe shares a kiss with Josh.Rose Byrne as Nat RedfernRafe Spall as Josh MossAnna Faris as ChloeSimon Baker as Guy HarrapStephen Merchant as DannyMinnie Driver as NaomiJason Flemyng as HughOlivia Colman as LindaJane Asher as DianaNigel Planer as BrianClare Higgins as Elaine