On a beach in the fictional town of Pigeon Creek, Alabama, 10-year-olds Jake Perry and Melanie Smooter inspect the result of lightning striking sand. Jake asserts that they will be married one day.

In the present day, Melanie is a successful New York fashion designer who has adopted the surname "Carmichael" to hide her poor Southern roots. After wealthy Andrew Hennings proposes, Melanie returns to Alabama to announce her engagement to her parents and finalize a divorce from her husband Jake, whom she married as a pregnant teenager and left after she miscarried their baby. Meanwhile, Kate Hennings, Andrew's mother and the Mayor of New York City, doubts Melanie's suitability to wed her son, whom she is grooming to run for President of the United States.

Melanie visits Jake, who has repeatedly refused to sign divorce papers over the years since she left for New York. After he orders her out of his house, Melanie empties Jake's checking account, hoping to spur him into ending the marriage. Angry, Jake leaves to meet some friends at a local bar without signing the papers. Melanie follows and gets drunk, insults her old school friends, and outs her longtime friend, Bobby Ray. Jake scolds her and takes her home, preventing her from driving drunk, and Melanie wakes to find the signed divorce papers on her bed.

Melanie goes to the Carmichael plantation and apologizes to Bobby Ray, whose family lives there. She is cornered there by Kate's assistant, sent to gather information on Melanie's background. Bobby Ray backs up her pretense that she is a relative and the family mansion is her childhood home. Melanie reconciles with her friends and learns that after she split with Jake, he had followed her to New York to win her back. Intimidated by the city and her success, he returned home to make something of himself first. She and Jake have a heart-to-heart, and Melanie realises why he never signed their divorce papers.

Andrew arrives to surprise Melanie, but upon learning her true background and that she never told him she was married, he angrily leaves. He later returns, saying he still wants to marry her, and the wedding is immediately set in motion. Melanie's New York friends arrive for the event. While visiting a nearby restaurant/resort with a glassblowing gallery, they admire the glass sculptures that are similar to ones they have seen in New York. Melanie realizes Jake is the artist and that he owns the resort.

During Melanie and Andrew's wedding at the Carmichael estate, a lawyer arrives and halts the ceremony. He has the divorce papers, which Melanie hadn't signed. Melanie confesses that she still loves Jake and cancels the wedding. She and Andrew wish each other well, though Kate berates Andrew and insults Melanie, her family, and the entire town, for which Melanie punches her in the face. Melanie finds Jake at the beach planting lightning rods in the sand during a rainstorm to create more glass sculptures. She tells him they are still married, they return to what would have been Melanie and Andrew's reception, and finally, have their first dance as husband and wife.

A mid-credits sequence shows that they have a baby daughter, Melanie continues to thrive as a designer, and Jake opens a "Deep South Glass" franchise in New York. Andrew is engaged to a girl named Erin Vanderbilt.