When Ben Parr and Kate Swanson accidentally meet at the top of the Empire State Building, they make a date for later that evening and each bring along their best friends, Larry and Connie, who instantly despise each other.
Ben and Kate try to go on their first official date. After much trouble finding a perfect night, Ben offers to come over to Kate’s place with plans to cook a romantic dinner. However, things do not go as planned when Connie's date never shows up, and Larry gets stuck there too watching a big game he has money on. Connie and Larry are constantly bickering further ruining the mood. When the swinging kitchen door hits Ben in the head, the four of them end up at the hospital and Ben and Kate admit defeat. Ben finally kicks Connie and Larry out so he and Kate can at least have a moment together.
It's Kate's birthday and Ben worries about what to get her, wanting the gift to be personal but without coming on too strong since their relationship is still in its early stages. Connie plans one of her annual grand birthday parties for Kate, this year’s theme is "Kate Gatsby" but Kate confides in Ben that she doesn’t like how big these parties have become and he encourages her to be honest with Connie. After a comment Kate makes, Ben realizes his gift for Kate is all wrong, and decides to surprise her with a private visit to the planetarium to the honor the first time that Kate and Connie met. However, all does not go as planned when Kate confronts Connie with her feelings about the party and the two have a huge falling out as a result. Meanwhile Larry recalls having been to one of Kate's birthday costume parties in the past and reveals he and Connie have made out.
Kate's younger sister Julia comes to New York to visit and we learn Kate is a very overprotective older sister. Ben desperately wants Julia to like him in hopes that she will give a good report to their parents. When Kate treats her sister like a child and drags her to see Annie, Ben gets stuck in the middle as Julia tries to get out of going by claiming Ben doesn’t feel well. Meanwhile, Connie and Larry play each others wingmen but end up pretending to have slept together to cover up for the fact that Kate's little sister actually sleeps with Larry.
Ben attempts to become better friends with Connie, baffled as to why she does not like him. Connie is not easy to impress but Ben tries by offering to help her watch Tiffany's kids. Although things do not go smoothly, after much effort on his part she finally admits she likes him better than all of Kate's other boyfriends. Meanwhile, in a moment of desperation, Larry calls an ex-girlfriend but quickly remembers how annoying she is. He decides to break up with her by "Munsching" her, which means he will just avoid her until eventually she breaks up with him. Kate hates that he blows girls off this way, and convinces him to be honest with her. However, in the end Kate realizes that Larry’s method may actually be kinder.
Ben works on a case with his ex Erin, and Kate pretends to be cool with it, but spies on him in the surveillance room. He figures it out and is glad that she cares enough to be jealous. Larry gets Kate to help him in the surveillance room so he can find his crush he calls "The White Whale". In his search, he also sees that Connie's new boyfriend is married. When Connie finds out the truth, Larry gives up his opportunity to meet his crush to hang with Connie and cheer her up.
Ben, Kate, Connie and Larry go on a road trip upstate to sell Ben's car. Ben and Kate are having a great time until Ben spontaneously tells Kate he loves her which causes her to crash Ben's car. Although things between Ben and Kate are awkward at first, Kate later admits she loves Ben too. Connie gets mad at Larry for flirting with a bartender to get free beer, but she does the same thing when she flirts with a cop to get out of a ticket. Larry calls the cop and invites him to Connie's motel room as a joke, but Connie ends up being surprised by the cop and agrees to go out with him.
Kate is stuck watching her boss' difficult dog for the weekend and she and Ben struggle to take care of it, each trying to prove how capable they are to one another. However, things get complicated when Kate loses the dog. Connie gets a weekend visit from Dennis, the police officer she has been dating, and Larry tries to befriend Dennis hoping he will revoke a ticket he had given Larry for not wearing a seatbelt. As a result, Connie and Larry fight over Dennis and in the end Dennis rejects them both.
Ben and Larry are forced to try and sign a young obnoxious client to the firm who is making them jump through hoops to get his business. Larry is even more annoyed when the client insists on meeting the same night as their Pub Quiz, an event in which he and Ben regularly participate and dominate. After they almost miss the quiz Larry tells off the client. He and Ben argue over this but ultimately they realize the bigger priority is their friendship. Fortunately, the client respects them for having the strength to stand up to him and signs with the firm. Connie has a bad back. In an effort to help cure her, Kate convinces her to go to a spa. During their massages, the masseuse compliments Connie and insults Kate. An annoyed Kate complains to Connie who is angry that Kate for once can’t let her be the center of attention. After arguing the two realize they are getting older, but happy with it.
Ben and Kate each worry that they are each not "fun" enough in the relationship and try to prove to the other that they are. However, after a spontaneous trip to Staten Island, things start to go very wrong. A series of events lead to Ben's clothes being stolen, along with his money and cell phone and they are challenged to make their way back to the city so that Ben can give a big speech at a legal conference, for which he is unprepared. Meanwhile, Larry runs into Connie in his office building and tags along to her meeting to pitch a children’s book in order to flirt with Tiffany. Ultimately, Ben and Kate are able to reach Larry and Connie who meet them in time to loan Ben one of Larry’s suits and Ben finds that even without his speech, the day with Kate is inspiration to help him wing it successfully.
Kate gets upset after Ben embarrasses her by telling everyone in their office building that she was wearing her new shower radio around her neck, not knowing it was meant to hang from the showerhead. After confronting Ben, she decides to point out a lot of Ben's flaws, which causes him get flustered in court, lose his case and ruin his perfect court record. Larry borrows Connie's camera and ends up losing it. Since the camera has lots of important pictures on it, Connie demands that Larry takes her to every place he visited the day before in an effort to find it. They eventually find the camera at Larry's ex-girlfriend's apartment, a single mother who has a son that Larry still spends time with. Connie is happy to get her camera back and is surprised to learn that there is more to Larry than she thought.
Ben and Larry help Tiffany pack up her apartment since she is getting divorced and moving to Florida. Larry uses the opportunity to finally make a move on her now that she's single, but ends up growing a conscience and realizes what Tiffany really needs a friend. Larry ruins a first edition of The Jungle Book that Tiffany gives Ben, only to find out that Tiffany's ex wants it back. Connie needs a new job now that Tiffany is moving away, and ends up working as a personal assistant to a celebrity chef. Things get awkward when Kate becomes friends with her boss. In the end, Tiffany decides not to move away and Connie goes back to working with Tiffany.
Ben plans to ask Kate to move in with him and all seems fine until Kate's high school boyfriend shows up and reveals that due to a clerical error, he and Kate are technically still married, the result of a dare on a trip to Niagra falls many years ago. This makes Ben realizes that there are plenty of things they don’t know about one another and they need to slow down. Connie's dad is getting married to a woman Connie hates so she invites Larry to the wedding as a fake date so her parents can freely be critical of him. But to her surprise, her dad likes Larry and he defends Connie to her new stepmother. Both Larry and Connie are rattled by the fact that they have some feelings for each other. However, things get complicated when Tiffany who is getting divorced, looks for comfort in Larry and kisses him and Connie witnesses the moment.