Bridget Jones's Diary opens with Bridget writing her New Year's resolutions, which focus on her desire to eat less, drink less, smoke less, and either find a boyfriend or learn to feel complete as a single woman. At the beginning of most diary entries, she records her current weight, the calories and drinks she has consumed, and the cigarettes she has smoked.
On the 1st of January, Bridget attends her parents' friends' Turkey Curry Buffet, where Una Alconbury tries to set her up with a successful divorced lawyer named Mark Darcy, who she used to play with when they were children. In the stilted middle-class atmosphere, Bridget dismisses him as dull because of the diamond-pattern V-neck sweater he wears, and they part without making plans to see each other again.
Bridget returns to her job in publishing and engages in an ongoing taboo flirtation with her boss, Daniel Cleaver. However, when he tells her during their first sexual encounter that this is "just a bit of fun," Bridget reprimands him for leading her on and doesn't go further. Meanwhile, Bridget's mother announces she is separating from Bridget's father, as she has started an affair with a Portuguese man named Julian—aka Julio. Bridget's mother also lands a job hosting a TV show called Suddenly Single, which features her interviewing other people who have recently left long-term relationships.
Bridget keeps Daniel interested by feigning disinterest in him, but she eventually begins flirting over the office messaging system. They have sex, but he acts distant in the weeks following. One night in April, he arrives at her flat while he is drunk and tells her he loves her. Reluctantly, Bridget lets Daniel up to use her toilet, and they end up spending the weekend together, now in a relationship. While Bridget's friends continue to complain about the "emotional fuckwittage" of the immature and commit-phobic men they date, Bridget is delighted to find that Daniel is turning out to be a model boyfriend.
In the summer, Bridget is displeased when Daniel shows no interest in going on weekend trips with her, and when they do finally drive out to the country, the trip doesn't satisfy her fantasies. Daniel cancels on Bridget the day he is supposed to accompany her to a party at the Alconburys. She goes alone but leaves early, returning to his flat unannounced. She discovers a naked American woman named Suki suntanning on his roof. It turns out that he has been having an affair. Soon after Bridget leaves him, Daniel tells her he is getting married to Suki.
After wallowing in despair, Bridget leaves her job at the publishing house and begins working in television. At a party at Mark Darcy's, Bridget is surprised when Mark asks her out to dinner. Although she pretends in her diary not to be interested in him, she is upset when he stands her up for the date. She later learns he was buzzing her in her flat but she couldn't hear because she was blow-drying her hair. He doesn't make another date with her, however.
Toward the end of the year, Bridget learns that the police are looking for her mother and her mother's boyfriend, Julio; it has come out that they've been conning friends out of thousands of pounds for deposits on non-existent time-share apartments in Portugal. The news comes on a night that Mark is at Bridget's for a group dinner; he jumps into action, making calls to sort out the issue.
In December, Bridget hopes to hear from Mark but doesn't. She learns he has gone to Portugal to track down Pam and Julio. Soon Bridget's mother returns to England in denial about the crime she has helped Julio commit and claiming that it is all a misunderstanding.
By Christmas, Bridget's parents are back together, sharing a bed again. Just as they are sitting down to Christmas lunch, Julio arrives at the house drunk and accuses Bridget's father of taking his woman. Pam takes Julio upstairs to a bedroom. Mark arrives with the police, and they arrest Julio.
Over a Christmas meal at a hotel, Mark explains that he knew he could exploit Julio's love for Bridget's mother by making him jealous enough to come back to the UK to face prosecution. Bridget asks why he has put so much effort into sorting out the problem, and Mark confesses that he is in love with Bridget. The two spend the night in a hotel room, giddy to be in love.
The novel ends with Bridget summarizing her year. Although she has only kept one resolution—to form a functional relationship with a responsible adult—it has nonetheless been an excellent year's progress toward her goals.
