When Alison Scott(Katherine Heigl) is promoted in E! Television, she goes to a night-club to celebrate with her older married sister Debbie(Leslie Mann). Alison meets the pothead reckless Ben Stone(Seth Rogen) and while having a small talk with Ben, Debbie's husband Pete(Paul Rudd) calls her to tell that their daughter has chicken pox. Debbie leaves the place but Allison stays with Ben, drinking and dancing along all night; completely wasted, they end up having a one night stand. Ben does not use condom and eight weeks later, Allison discovers that she is pregnant. She calls Ben and they decide to try to stay together and have the baby. However, Ben needs to grow-up first to raise a family of his own.