Ben Stone (Seth Rogen) is laid-back and sardonic. He lives off funds received in compensation for an injury and sporadically works on a Mr. Skin-like website with his roommates, in between smoking marijuana or going off with them at theme parks such as Knott's Berry Farm. Alison Scott (Katherine Heigl) is a career-minded woman who has just been given an on-air role with E!, and is living in the pool house with her sister Debbie's (Leslie Mann) family. While celebrating her promotion, Alison meets Ben at a local nightclub. After a night of drinking, they end up having sex. Due to a misunderstanding, they do not use protection: Alison uses the phrase "Just do it already" to encourage Ben to put the condom on, but he misinterprets this to mean to dispense without even using one. The following morning, they quickly learn over breakfast that they have little in common and go their separate ways, which leaves Ben visibly upset.Eight weeks later, Alison experiences morning sickness during an interview with James Franco, and discovers she is pregnant. She contacts Ben for the first time since their one-night stand to tell him. Although insensitive at first, Ben says he will be there to support Alison. While he is still unsure about being a parent, his father (Harold Ramis) is excited. Alison's mother (Joanna Kerns) tries to persuade her daughter to have an abortion, but Alison decides to keep the child.Later, Alison and Ben decide to give their relationship a chance. The couple's efforts include Ben making an awkward marriage proposal with an empty ring box, promising to get her one someday. Alison thinks it is too early to think about marriage, because she is more concerned with hiding the pregnancy from her boss, who asked her to be "firm" and "tight" for the cameras.After a somewhat promising beginning, tensions surface in the relationship. Alison is increasingly worried about Ben's lack of responsibility and commitment and has doubts about the longevity of their relationship. These thoughts are due to her sister's unhappy marriage. Debbie's husband, Pete (Paul Rudd), works as a talent scout for rock bands, but he leaves at odd hours in the night, which makes her suspect he is having an affair. Upon investigating, she learns that he is actually part of a fantasy baseball draft, which he explains he participates in to be free from Debbie's controlling manner. This results in their separation, and when Ben expresses amusement at Pete's deception, it leads to a heated argument with Alison as they drive to her doctor. Angered, she ejects him from her car and abandons him in the middle of nowhere, leading to their own breakup.Ben and Pete decide to go on a road trip to Las Vegas. Under the heavy influence of psychedelic mushrooms, they realize their loss and decide to take responsibility for their relationships. Simultaneously, Debbie drags a timid Alison out partying with her, but they are refused admission to a nightclub by its apologetic bouncer (Craig Robinson) on account of Debbie's age and Alison's pregnancy, leading to Debbie's tearful laments about her life and her desire to have Pete back. They reconcile at their daughter's birthday party, but when Ben tries to work things out with Alison, she is still reluctant to get back together with him.Alison's boss finds out about her pregnancy and sees an opportunity to boost ratings with female viewers by having Alison interview pregnant celebrities. After a talk with his father, Ben decides to take responsibility and goes to great effort to change his ways, including moving out of his friends' house, getting an office job as a web designer and an apartment with a baby's room. He also starts reading the pregnancy books that he had purchased early on. When Alison goes into labor and is not able to contact her doctor, she calls Ben, as Debbie and Pete are out of town on a trip. Ben discovers that the gynecologist they had been seeing (Loudon Wainwright) is out of town, despite having assured them that he never took vacations. Ben calls him and leaves an assertive voicemail, threatening murder.During labor, Alison apologizes for doubting Ben's commitment and admits that she never thought the man who got her pregnant would be the right one for her. When Debbie and Pete arrive at the hospital, Ben adamantly refuses to allow her to be at Alison's side, insisting that that is his place. Debbie is both furious and impressed that Ben took charge of the situation and begins to change her formerly negative opinion about him. The couple welcomes the birth of a baby girl (a boy in the alternative ending) and settle down happily together in a new apartment in Los Angeles.