Jane Nichols has been a bridesmaid for twenty-seven weddings. One night when she is attending two weddings almost simultaneously, she meets Kevin Doyle, who disgusts her with his cynical views of marriage. Kevin and Jane share a cab home, where she forgets her day planner. Kevin snoops through the planner and resolves to return it to Jane. Meanwhile, Jane's sister Tess returns from a trip to Europe and quickly falls in love with Jane's boss George, on whom Jane has an unrequited crush. Tess feigns interest in George's passion for the outdoors and animal rights, and their courtship progresses rapidly. Soon, they announce plans to marry in only three weeks. Tess enlists Jane to be the wedding planner.

Tess and George's nuptials will be featured in the newspaper's Commitments section. The reporter turns out to be Kevin, who writes wedding announcements under the pseudonym Malcolm. Kevin successfully returns Jane's planner but decides to use the contents as material for a piece on the "perennial bridesmaid" as a stepping stone to more serious journalistic pursuits. Jane is unaware of Kevin's intentions. During their interview about Tess's wedding, Jane shows him the 27 bridesmaids' dresses in her closet. He takes pictures of her wearing all 27 dresses and submits the photos and his article about Jane for publication. As Kevin and Jane grow closer, he has second thoughts and begs his editor to hold off on publishing the article.

Kevin accompanies Jane on a wedding-related errand in Rhinebeck, New York. When Jane's reckless driving causes the car to skid off the road and become stuck, the two seek refuge at a local bar, and enjoy drunken revelry while singing "Bennie and the Jets". Kevin and Jane kiss and have sex in the car. That day, Kevin's editor runs the article about Jane on the front page of the Commitments section. Jane is betrayed and is furious at Kevin. Tess then gets angry at Jane for giving Kevin material about her, whom he describes as a bridezilla. The fight escalates when Jane learns that Tess altered their late mother's wedding dress to make her own gown (which Jane wanted for herself), the last straw on Tess's string of lies to George and demands on Jane.

Despite the fight, Tess still asks Jane to make a slideshow for her engagement party. Jane exacts her revenge by sharing pictures of Tess flirting with other men in the past, eating ribs, and holding a cat by the tail - in short, illustrating her numerous lies to George. After Pedro, the child that George mentors, reveals that Tess had him cleaning George's apartment for money, George breaks off the engagement.

Later at work, George tells Jane that he appreciates her because she never says no. Remembering that Kevin once said the same thing as criticism, Jane quits and admits she only stayed at the job because she was in love with George. She discovers after an experimental kiss that she no longer loves George. She finds Kevin at a wedding he is covering and announces her love for him.

One year later, Jane and Kevin are getting married. George and Tess meet at the ceremony and hope for a second chance. All 27 brides serve as Jane's bridesmaids, wearing the dresses that she once wore as their bridesmaid.