Sadie Jones (Mandy Moore) has always longed to marry the man of her dreams in her family church. Though she has found her lifetime companion in Ben Murphy (John Krasinski), Sadie is distressed to learn that St. Augustine's has only one wedding slot available over the next two years, though after re-checking their planning book, they find that the wedding can be held in three weeks. While Sadie and Ben do qualify for the slot, the church's eccentric minister, Reverend Frank (Robin Williams), will not wed the couple until they agree to attend his prenuptial course (shortened, due to the new date, from three months to three weeks). Now as their wedding date draws near, Sadie and Ben must now follow all of Reverend Frank's rules, attend his unusual classes, and complete a series of homework assignments designed specifically to irritate one another in order to get past puppy love and ensure that their union will have a sound foundation. To Ben's dismay, one of Frank's rules is no pre-marital sex. On behalf of Frank, his young assistant breaks into the couple's house and bugs it. Thus, Frank and his assistant can listen to all conversations, though Frank doesn't let his assistant listen to the adult parts. Ben discovers the microphone/ transmitter but does not tell Sadie, for fear she will accuse him of lying and planting the bug himself. In one part of the course, the couple has to care for twin "creepy robot" babies. They get on Ben's last nerve and he destroys one, to the horror of bystanders in a department store. Shortly before the wedding, Sadie changes her mind, among other things because Ben has not prepared marriage vows as Frank instructed them to do, but instead drew a truck. Sadie goes on vacation to Jamaica, their slated honeymoon destination. Ben and Frank travel there too. Ben writes his vows on the sands of the beach to impress Sadie and they reconcile, and Frank marries them there.