The film starts with Bridget Jones (Renee Zellweger) sitting in her London apartment listening to "All By Myself" while holding a birthday cupcake. She is turning 43, and she finds herself alone. Instead of moping, she changes the song to "Jump Around" and starts getting funky.
Bridget narrates details how her birthday really began. First, she gets a phone call from her mom Pamela (Gemma Jones) that now that she is in her early 40's, her time to have a baby is running out. She goes to a church to the funeral of her former lover Daniel Cleaver (Hugh Grant). A number of his former girlfriends are in attendance as well. As she sits with her friends, Bridget spots her old boyfriend Mark Darcy (Colin Firth) with his new wife Camilla (Agni Scott). After the funeral, Bridget and Mark run into each other to catch up for a brief, awkward moment as he introduces Bridget and Camila to each other, and then they part ways.
Bridget currently works as a television producer for her anchor friend Miranda (Sarah Solemani). Bridget speaks to Miranda as she interviews a guest, but Bridget takes a phone call during the segment, so Miranda repeats whatever Bridget says on the phone, which in this case makes Miranda's interview regarding the death of a fascist dictator seem unintentionally sympathetic. To complicate things further, a younger manager named Alice (Kate O'Flynn) is hired, and it's said that she's been known to fire people older than her.
Miranda invites Bridget to go out for her birthday, but she would rather spend it with her other close friends. Unfortunately, they're all too busy with their own kids. Not ready to get discouraged, Bridget decides to take Miranda up on her offer, and she joins her on a trip to a music festival. When they arrive, Miranda tells Bridget to sleep with the first man she meets. Bridget falls face-first in the mud, and she meets a handsome stranger named Jack Qwant (Patrick Dempsey), who helps her on her feet.
At night, Bridget and Miranda get drunk and bump into Ed Sheeran to take a picture for them, though he initially thinks they want one with him, but they don't even recognize him at first. They later attend his concert, with Miranda rolling around in a giant hamster ball. Bridget stumbles into a yurt that she thinks is hers and Miranda's, but it's Jack's. After some flirtation, they have sex. In the morning, Bridget wakes up in bed alone.
Upon returning home, Bridget attends the christening of her friend Jude's (Shirley Henderson) child. She runs into Mark again as they are both asked to be the child's godparents. Mark mentions that he and Camila are divorcing. They attend a party later, leading to them reconnecting and eventually having sex. Afterwards, Mark mentions he is traveling the next day. He wakes up the next morning to find a note from Bridget saying they shouldn't continue this relationship, citing reasons as to why it doesn't work between them.
A few weeks later, Bridget notices she is gaining weight. She talks to her friend Shazza (Sally Phillips) over her recent flings and mentions using expired vegan condoms. Bridget takes a pregnancy test at work with Miranda and discovers that she is pregnant. Although excited, she visits OB/GYN Dr. Rawlings (Emma Thompson) and realizes she has no clue as to who the father is.
Alice demands more news stories worthy of viewers at work. When Miranda discovers that Jack is a billionaire mogul that created a successful dating site (Qwantify Love), she tells Bridget and plans to have him on the show as a guest, but also to try and sneak a hair sample off the man to use in a DNA test. Jack appears on the show, thinking he's there to talk about his book and algorithms on finding love, but the ladies try to keep him on interesting topics, which makes the interview come off clumsy. After the show, Bridget and Jack meet again, with her admitting that she is pregnant, but never mentioning that the baby could also belong to Mark. Luckily, Bridget manages to get some hair from Jack taken by the stylist.
Bridget talks to her dad Colin (Jim Broadbent) about her situation. She apologizes for putting herself in this predicament, but Colin is very understanding.
Bridget goes to find Mark in a courthouse as he is representing a group of women in a highly publicized case. Later, she tells Mark about her pregnancy, but she fails to mention Jack, so Mark thinks the baby is his as well.
Jack starts to get involved in Bridget's life as he tries to support what he thinks is his baby. She also decides not to do the DNA test right away as she worries she may suffer a miscarriage. She even makes Dr. Rawlings to do appointments twice with both possible fathers to keep stringing them along.
Bridget's job holds an event to introduce new things in their work. She invites Jack but is surprised when Mark shows up as well. Bridget's presentation turns into a disaster when they record men mooning the camera, and the show suffers technical difficulties. After this, Mark and Jack end up meeting and get on as they admire each other's work, but Bridget is forced to admit to both men over dinner that they are potential fathers to her child, and she doesn't know who the legitimate father is. Jack doesn't seem to have a problem with this, but Mark leaves the restaurant feeling upset.
Mark tries out Jack's dating site and tries to see if he's a match with Bridget. Their result is less than 10%. He puts Jack and Bridget together and sees they have over 90% in compatibility, to his chagrin.
Over the next few months, Jack and Mark help Bridget out however they can as her pregnancy moves along, such as attending a pre-natal class where both men are mistaken as a gay couple with Bridget as their surrogate. Mark later finds that Jack brought Bridget to the hospital when she was experiencing cramps, leading to a heated discussion between the two where Jack says he wasn't "dressed for the occasion" when he hooked up with Bridget. Thinking Jack might really be the father, Mark steps out of the picture, albeit heartbroken.
Jack attempts to move his relationship with Bridget forward by suggesting they move in together so that he can take care of the baby with her. Bridget notes that it may get complicated if the baby turns out to be Mark's. Jack admits that he may have been responsible for Mark leaving when he suggested that the baby could be his (Jack's). Bridget goes to Mark's house to talk to him but leaves when she sees Camila there.
Bridget reaches her ninth month of pregnancy. At work, Miranda is supposed to interview a Korean general, but Bridget brings her the man's chauffeur instead, leading to a thoroughly unpleasant interview. Alice is preparing to fire Bridget for her screw-ups, but Bridget tries to take it a step further and quit herself. She immediately regrets it afterwards.
Now jobless and upset, Bridget goes out to get food. Her cards gets retained by an ATM machine, and she is left alone in the rain. Mark shows up and helps her. The two spend more time together, and he tells Bridget that he really is divorcing Camila, and she was there to pick up her stuff. They are about to kiss, but Bridget's water breaks. Mark's phone starts ringing, and, thinking it's a work thing, he smashes his phone out of what he thinks is a bold romantic gesture...except they needed the phone to call a cab.
Bridget and Mark ride a carriage to the hospital, only to get stuck in a traffic jam. Mark decides to carry Bridget to the hospital since the contractions are making walking difficult, but he starts to get tired because Bridget is so heavy. Jack shows up and helps take Bridget to the hospital. Bridget's parents and her work friends learn that she is going into labor, and they all celebrate.
At the hospital, both men try helping Bridget, but she leans more toward Mark. He promises to love her and the baby, no matter whose it is. Bridget ends up giving birth to a healthy baby boy. Her parents and friends show up to her room after. Dr. Rawlings then calls the men in for the DNA test. They wish each other good luck.
A year later, Bridget is getting married. As she walks up to the altar, Jack steps out with Bridget's son, William. It turns out she is really marrying Mark, and the baby is his. They all head onto the reception afterwards, with Bridget now feeling more fulfilled with her life.
The final shot shows a newspaper headline that states that Daniel Cleaver is, in fact, alive.