Kate (Olivia Wilde) and Luke (Jake Johnson) are co-workers at Chicago craft brewery Revolution Brewing, where they spend their days drinking and flirting. They are perfect for each other, except that they are both in relationships: Kate is with Chris (Ron Livingston), and Luke is with Jill (Anna Kendrick). Jill presses Luke to see if he is yet ready to talk about marriage, which he promises he will be sometime soon.
Luke is busy brewing beer and Kate handles phone calls to set up the anniversary party for the brewery. During lunch they agree to go out for drinks after dinner with their co-workers from the brewery. While all are drinking at the bar, Luke's fiancee, Jill, arrives. After the bar, Kate rides her bike over to the apartment of her socially awkward, but extremely nice boyfriend where he gives her dinner and a copy of Rabbit, Run. Later, at the brewery party, Chris invites Luke and Jill to join them for a trip to his family's cottage.
During the trip Jill and Chris go for a long hike in the woods, where they end up kissing. Luke and Kate spend the whole trip drinking and staying up late alone together, including a bonfire on the beach. Kate invites Luke to go skinny dipping before stripping off her clothes and running into the lake.
After the group returns home, Chris decides he needs to talk to Kate and the two break up. Newly single Kate insists the whole brewery crew go out the next evening for drinks, to celebrate her 'singleness', and that night she ends up sleeping with a co-worker, Dave. When Luke hears about this he is angry at both Dave and Kate, and spends the whole day pissed off, snapping at both of them, but eventually apologizes for his behavior. Jill decides to go away for a week with some friends from college. During this trip, Luke and Kate go out for dinner and fall asleep together on the couch. Luke agrees to help Kate clean out her old apartment and move the next day, and at the end of the day Luke falls asleep on the bed. Kate joins him.
During the next morning, Luke invites Kate to dinner to celebrate the move, but then cuts his hand while helping her move the couch. Kate is squeamish about the blood and unhelpful. An impatient driver, blocked by the moving van, yells at them and starts a fight with Luke. After Luke ends up with a fat lip, Kate calls Dave and another co-worker to help finish the move. Dave invites Kate to go out for drinks, and she tells Luke that she wants to go, rather than going to dinner. Luke is annoyed, and an argument ensues.
Returning home, Luke finds Jill has returned home early, and she is crying. She confesses to kissing Chris while at the cottage, tells Luke how guilty she feels about what happened, and that she really loves him and wants to marry him. Luke forgives Jill and lets her know that he still loves her and wants to marry her. The next day at work Kate and Luke awkwardly interact and then eventually end up sitting together at lunch. They each offer one another food items before they crack a smile, and drink a beer.