Perpetual adolescent Ben Stone and his four roommates spend their days in the San Fernando Valley smoking marijuana, playing games and wasting time. In another part of Los Angeles, hard-working E! Television producer Alison Scott lives in a guest house next to her sister Debbie, brother-in-law Pete and two small nieces. While Ben and his roommates discuss their proposed website, “Flesh of the Stars,” which catalogs celebrity nude scenes, Alison is called into her boss Jack’s office and is thrilled to learn that she is being promoted to onscreen celebrity interviewer. Debbie and Alison celebrate by going to a dance club, where the doorman lets the attractive sisters in ahead of everyone in line. Ben and his roommates are there and when Alison goes to get drinks, she meets Ben, who tells her that the bartender is ignoring everyone and solves the problem by taking two beers and giving one to her. Then, realizing she needs another beer for Debbie, Ben gives Alison his. Debbie has to leave, but Alison stays, dancing and drinking with Ben until they go back to her place and begin to have sex. As Ben struggles with a condom, Alison impatiently says “just do it,” which he interprets as “forget the condom,” and they have unprotected sex. The next morning, Alison is regretful and Ben barely remembers the night, but after recovering from his shock at being awake at 7:30, he suggests they have breakfast before she goes to work. In a coffee shop, the gulf between Alison and Ben widens as he elaborates on his marijuana habit, disparages celebrity interviews and explains his website. When Ben asks to see Alison again, she is clearly not interested and when she tells her sister about the incident, Debbie is relieved that Ben used a condom. Eight weeks later, Alison is taping an interview with James Franco when she has to run off camera to vomit, and when she gets sick again later, the suggestion of pregnancy is made and she begins to worry. Debbie and Alison buy several home pregnancy tests, which all give positive results, so they email Ben to arrange a date to talk. Thinking that Alison is interested in sex, Ben and his roommates scream in triumph. Attempting to get to know Ben over sushi, Alison learns that he is living on an insurance settlement rather than working and that he is a Canadian residing in the United States illegally. When Alison announces the likelihood of her pregnancy to Ben, they argue about the misunderstanding over the condom, but he agrees to go with her to the gynecologist and tries to comfort Alison when the doctor confirms she is expecting. The news is met by criticism from Ben’s roommates and Alison’s mother, who suggests an abortion to save her career, warning that her that her appearance will soon undermine her decision to keep her pregnancy from her employers. However, Ben’s father tells him to roll with it and that night, when Alison tells Ben she will keep the baby, he offers to do whatever is needed. After discussing their situation, Ben and Alison go to a mall where he buys baby books and they stroll holding hands. Later that night, Ben and Alison agree to pursue their relationship for the sake of the baby and he assures her that he will not desert her. The next morning, Alison introduces Ben to her nieces as her boyfriend, and as Debbie and Alison watch him playing with the children, Debbie agrees to give him a chance. Needing money, Ben asks his roommates when the website will be running and they estimate three months. Interviewing gynecologists, Alison rejects the rigid Dr. Kuni and several other inappropriate doctors before choosing Dr. Howard, when he assures her that he will personally deliver her child. While shopping for cribs, Alison tells Debbie that Ben knows the sex of the baby, but that she wants to be surprised. After Ben suggests using a crib from the alley behind his house to save money, Debbie offers to buy them one and Alison is put off by Ben’s acceptance of the extravagant gift. As Alison is leaving the store, some old friends spot her, and Ben embarrasses her by disclosing her condition. Ben later attributes her discomfort to the fact that she is unmarried and asks Pete how much wedding rings cost. Their discussion leads Ben to explain his website, and Pete informs him that a similar one already exists. Ben’s roommates are devastated by the news, but postpone working on a new idea because Spider-Man 3 is on television. When Ben proposes to Alison with an empty ring box, promising to fill it someday, she is touched, but replies they have not known each other long enough to marry. While at Ben’s house one night, Alison is angered when his fear of hurting the baby prevents him from making love to her and later, she wakes up alone during an earthquake because Ben has left to smoke marijuana. Alison begins to doubt Ben’s ability to take care of her and the baby, and is further dismayed to find the baby books he promised to read are unopened. When Debbie suspects Pete of cheating, Ben reluctantly joins the women in following Pete to a house where they find him playing fantasy baseball with some men. Pete is incredulous when Debbie tearfully explains that wanting to spend time away from his family is worse than adultery and tells him to move out. The next day, on the way to the gynecologist, Alison criticizes Pete and when Ben takes his side, they argue and she throws him out of the car. Ben arrives at the doctor’s office, furious after having to walk there, and their fight culminates with him disclosing that the baby is a girl. Later, while commiserating in a bar, Ben and Pete decide to go to Las Vegas, while at home Debbie insists to Alison that they go out to have some fun. Arriving in Las Vegas, Ben and Pete take hallucinogenic mushrooms and go to see Cirque de Soleil, but Ben panics and runs back to their room. In Hollywood, when Debbie and Alison are refused entrance to a dance club because Debbie is too old and Alison is pregnant, Debbie weeps over her spent youth and her future alone. Ben and Pete’s altered state leads them into emotional recriminations over their treatment of their women, and they decide to go home. Back in L.A., Ben tries to reconcile with Alison, but she maintains they are not right for each other. Devastated, Ben pleads for advice from his father, who tells him that nothing will work until he accepts responsibility for himself. Taking this to heart, Ben gets a job and moves into an apartment where he prepares a nursery. In Alison’s eighth month of pregnancy, Jack finally broaches the subject with her, announcing to her great relief that the network approves and will showcase her condition. Alison’s family is gone for the weekend when she begins to have contractions and calls Dr. Howard, but gets his on-call substitute. Unable to drive, Alison desperately calls Ben and asks him to take her to the hospital and when he arrives, he calls Dr. Howard to learn that he is out of town. After skillfully breaking this news to Alison, Ben asks such specific questions to ascertain how much time they have before the baby arrives that Alison realizes he has read the baby books. Ben drives Alison to the hospital where a very cranky Dr. Kuni arrives to deliver their baby, and when Alison insists on using her intended birth plan, he walks out on her. Ben stops Dr. Kuni, persuading him to help Alison and the doctor returns and apologizes to her. After Alison and Ben agree that they are right for each other, Pete and Debbie arrive, and when Debbie dismisses Ben, he orders her out of the room and she joins Pete and the roommates in the waiting room. After some complications and a painful delivery, the baby is born and as Alison sleeps, Ben tells his child about the miscommunication that led to her conception, saying it was the smartest thing he ever did. Later he drives his new family home.