Ambitious Los Angeles reporter Alison Scott has just been given an on-air role with E! and lives in the guest house of her sister Debbie's family. Ben Stone is an immature and wise-cracking Canadian slacker who lives off injury compensation funds and sparsely works on a celebrity porn website with his stoner roommates. While celebrating her promotion, Alison meets Ben at a local nightclub. After a night of heavy drinking, they end up having sex without protection due to a misunderstanding. The following morning, they learn over breakfast that they have nothing in common so go their separate ways.

Eight weeks later, when Alison experiences morning sickness while interviewing James Franco she realizes she could be pregnant. After taking multiple pregnancy tests, she is shocked to find out she is pregnant. She contacts Ben for the first time since their one-night stand to tell him. Although abrasive at first, he says he will support Alison. Although he is still unsure about being a parent, his father is overjoyed. Alison's mother tries to persuade her to have an abortion, but Alison decides to keep the child.

Later, Alison and Ben decide to give the relationship a chance. The couple's efforts include Ben making a marriage proposal with an empty ring box, promising to get her a ring someday. Alison thinks it's too early to think about marriage, as she is more concerned with hiding the pregnancy from her bosses, fearing they will fire her if they ever found out. After a positive beginning, tempers flare in the relationship.

Alison is increasingly worried about Ben's lack of support and understanding, and has doubts about their relationship's longevity. These thoughts are due to her sister's loveless marriage. Debbie's husband, Pete, works as a talent scout for rock bands, but leaves at strange hours in the night, making her suspect he is having an affair. Upon investigating, she learns that he is actually part of a fantasy baseball draft, and that he has been doing other activities such as going to the movies on his own, which he explains he participates in to be free from Debbie's manipulative manner.

As a result they separate, and when Ben expresses pride in Pete's deception, it leads to an argument with Alison as they drive to her doctor. Furious, she ejects him from her car, abandoning him in the middle of a busy street. He tracks her down to her appointment and they have another argument, leading to their own breakup.

Ben and Pete go on a road trip to Las Vegas. Under the influence of psychedelic mushrooms, they realize their loss and decide to try and save their relationships. Simultaneously, Debbie drags a nervous Alison out partying with her, but they are refused admission to a nightclub by its sympathetic bouncer 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 refuses to get back together. Alison's boss finds out about her pregnancy and sees an opportunity to boost ratings with female viewers by having her interview pregnant celebrities. After a talk with his father, Ben decides to take responsibility and goes to great effort to mature, including obtaining his own apartment, getting an office job as a web designer, and reading pregnancy books.