Bridget Jones's Diary portrays a year in the life of 30-something, career-minded Bridget Jones. Bridget is a self-involved woman concerned with her weight, appearance and securing a boyfriend. The book is written as a diary and tracks Bridget's life during a twelve-month span, beginning with her New Year's Resolutions. Her resolutions include quitting smoking, drinking less, losing weight, finding a boyfriend, getting a better career, and not dating emotionally unavailable men.

The chapters of the book represent the twelve months of the year. Each chapter is broken up into daily entries. Most of the daily entries begin with a tally of Bridget's weight, alcohol consumption, cigarettes smoked and parenthetical comments on her progress. Because the book is written like a diary the language is very frank and honest. The author has even come up with a set of abbreviations, which the reader easily catches on to such as "v." for "very" or "v.g." for "very good."

Bridget's year begins with her having a crush on her boss, while her parents try to fix her up with Mark Darcy, who is recently divorced and back from America. Bridget has no luck with or attraction to Mark Darcy. Her flirtations with her boss, Daniel Cleaver, result in his asking her out for a date. Meanwhile, her parents are going through marital troubles of their own.

As the story continues, Bridget and Daniel date sporadically. He is unpredictable and hard for Bridget to understand. She frequently is frustrated with him and swears never to sleep with him again. Inevitably, he does something endearing, and she ends up seeing him again. Bridget also has chance encounters with Mark Darcy, in which she ends up saying or doing something she deems foolish and unattractive.

Bridget's mother leaves her father and runs off with a Portuguese man named Julio, who's younger, wilder and more handsome than Bridget's father. After a few other trysts, Bridget's mother decides that men are not what she's after. She wants the career she never had, so she gets a job in television.

Bridget and Daniel settle into a relationship and spend almost every weekend together. They even plan a weekend away in the country together. Bridget is happy to have a boyfriend, even if they do spend each summer Sunday watching cricket with the blinds closed and the room dark. One day, Bridget pops by Daniel's place unexpectedly and finds a naked, beautiful American sunning herself on his terrace. Bridget is devastated.

Now that Daniel is out of her life, Bridget focuses on her career. Her mother gets her an interview with a television station. Bridget begins working in television and finds much success. Her shining moment is a coveted exclusive interview, awarded by her friend Mark Darcy, with his star client.

After a party at his house, Mark Darcy asks Bridget out for a date. They set plans for dinner, but Bridget does not hear the doorbell ring when he comes, and they stand each other up for their first date. Bridget invites him to a dinner party at her flat, which turns out to be a culinary disaster. The same night, she finds out that her mother is a fugitive, along with her boyfriend. Julio has stolen a lot of money from family friends, including Mark Darcy's parents, in a time-share scam. After that night, Bridget waits for Mark to call her, but he doesn't.

A few days later, she hears from Mark. He's in Portugal trying to get her mother and the money back. Her mother returns to London and some of the money is recovered, although Julio is still missing. December begins, as does the Christmas season, and Bridget is in full party mode. She attends numerous parties, lunches and gatherings with her friends. Through it all she hears nothing from Mark and figures her horrible cooking and criminal family have scared him off.

On Christmas Day, Julio appears at Bridget's family's home with Mark Darcy in hot pursuit. The police are called, and Julio is arrested. Mark Darcy sweeps Bridget away and declares his love to her. The book ends with a summary of Bridget's progress on her New Year's Resolutions, only one of which she kept. She found a wonderful boyfriend.