Taking place a few years after the events of the Sex and the City television series, Carrie is now in a committed relationship with Mr. Big, and they are viewing apartments with plans to move in together. Carrie falls in love with a penthouse far from their price range. Big offers to pay for it. Carrie offers to sell her own apartment, although she also fears that she would have no legal rights to their home in case they separate, as they are not married. To quell her fears, Big suggests that they marry. Charlotte hires her longtime friend, Anthony Marantino, as the wedding planner.

Carrie shows Anthony and Charlotte the vintage suit she plans to marry Big in, but no one except Carrie loves it. Carrie is asked by her Vogue editor Enid to do a spread for Vogue called "The Last Single Girl". She gets to dress up in fancy couture gowns and is gifted a gown from Vivienne Westwood, which she decides to wear instead of her suit. The ensuing publicity of her engagement blows up her small wedding to a big affair, which makes Big increasingly uncomfortable.

Miranda, who has settled down in Brooklyn with Steve to raise Brady together, has been so busy with work, that she has not had sex with Steve in six months. When Steve confesses he has cheated on her, Miranda is devastated and immediately leaves him. At Carrie and Big's rehearsal dinner, Steve tries to reconcile with Miranda, but she is still upset and tells Big bluntly that marriage ruins everything. On the wedding day, a stressed-out Big cannot go through with the ceremony. As a devastated Carrie flees the wedding, Big quickly realizes his mistake and catches up with Carrie, who furiously attacks him with her bouquet in the middle of a one-way street.

Miranda tells Charlotte that she may have upset Big, and she wants to tell Carrie. Charlotte tells her not to, as Big has always had doubts about marriage. To console Carrie, the women take her on the honeymoon that she had booked to Mexico, where they de-stress and collect themselves. Upon returning to New York, Carrie hires an assistant, Louise, to help her manage her life. She gets her apartment back, and Louise helps her put up her website. Charlotte, who is married to Harry and has adopted a Chinese girl named Lily, learns she’s pregnant. She is fearful that something might happen to her baby, but Carrie reassures her.

On Valentine's Day, Carrie and Miranda have dinner, where she tells Miranda that reading the Vogue article about her and Big's engagement made her realize that she had become so consumed with the wedding that it was not about her and Big anymore; it was all about her. Miranda then confesses to Carrie about what happened at the rehearsal dinner. Carrie is furious that she had ruined her wedding. After a few days, Miranda gets Carrie to talk to her, and begs for her forgiveness; Carrie asks that she does the same for Steve. Miranda attends couples counseling with Steve and they eventually reconcile.