Allison Scott (Heigl) is an up-and-coming entertainment journalist whose 24-year-old life is on the fast track. But it gets seriously derailed when a drunken one-night-stand with slacker Ben Stone (Rogen) results in an unexpected pregnancy. Faced with the prospect of going it alone or getting to know the baby's father, Allison decides to give the lovable layabout a chance. An overgrown kid with no desire to settle down, Ben learns he has a big decision to make with his kid's mother-to-be: will he run or stay? Courting a woman you've just Knocked Up, however, proves to be difficult when the two try their hands at dating...and more. As they discover more about one another, it becomes painfully obvious that they're not the soul mates they'd hoped they might be. With Allison's sister Debbie (Mann) and hen-picked brother-in-law Pete (Rudd) the only parenting role models the young lovers have, things get even more confusing. Should they raise the baby together? What makes a happy lifetime partnership after all? A couple of drinks and one wild night later, they've got nine confusing months to figure it out...