On her 43rd birthday, Bridget Jones is awoken by her mother who reminds her that her time to have children is running out. She goes to attend the funeral of Daniel Cleaver, who is presumed dead after a plane crash. While there she sees her ex, Mark Darcy (Colin Firth), and his new wife. They bump into one another somewhat awkwardly and then go their separate ways.
Bridget now works as a television producer and is close friends with the anchor of the show she runs, Miranda (Sarah Solemani), who offers to take her out for her birthday. Bridget turns down her offer to spend time with her old group of friends, but is heartbroken when they cancel on her due to commitments to their children. Deciding to celebrate her single life Bridget accepts Miranda's offer to take her away for the weekend. They go to a music festival where Miranda challenges her to sleep with the first man she meets. Falling in the mud, Bridget is helped out by a random stranger. After attending an Ed Sheeran concert a drunk Bridget crawls into a yurt she thinks belongs to her and Miranda, but actually belongs to the handsome stranger she met earlier. The two have sex and in the morning Bridget tries to not be disappointed when she wakes up alone.
Returning home Bridget goes to the christening of Jude's youngest child where she is the godmother and Mark has been asked to be the godfather. Mark tells her that he and his wife are planning on divorcing and Bridget has sex with him. When Mark reveals he is travelling the next day Bridget is reminded that he had always put work before their relationship and leaves in the morning before he wakes up, leaving behind a note telling him that reconnecting with him is too painful.
Bridget finds that she is gaining weight. Shazza suggests that Bridget might be pregnant, especially when she realises that Bridget used old, expired condoms when having sex. Taking a pregnancy test at work, Bridget confirms she is pregnant and decides that she wants to keep the baby as it is her last chance to have a child. After a visit to the obstetrician and gynecologist, Dr. Rawlings, she realises that she does not know who the father is and doesn't even have a way of contacting Jack, the handsome stranger. However Miranda spies him on TV where they realise that Jack Qwant is a billionaire and the inventor of a dating site that has an algorithm to match couples together. Miranda conspires with Bridget to have Jack on the show to take hair samples so that Bridget will be able to test it against the DNA of her baby to either confirm or eliminate Jack as the father. At the taping the hair and makeup artist is able to nab samples of Jack's hair for Bridget but Jack recognises Bridget and she decides to tell him that she is pregnant and tells him he is the father, without mentioning Mark Darcy. Initially taken aback Jack decides to throw himself into the role of supporting Bridget.
Feeling things are going too quickly with Jack, Bridget decides to go forward and tell Mark. However Mark is so thrilled at the news that he is about to be a father that Bridget does not tell him about Jack. Bridget also decides not to go forward with testing the DNA while her child is still in the womb as she is terrified by the possibility that she might miscarry. She persuades Dr. Rawlings to go through appointments twice, once with Jack and another time with Mark.
Bridget invites Jack to a work event and is startled when Mark shows up as well. The two men immediately take a liking to one another, finding the other's accomplishments impressive. They go out to dinner where Bridget finally comes clean and tells them that she is unsure who the father is. Jack takes the news well, telling Bridget that the child is the priority. Mark Darcy is upset and walks out of the restaurant.
At an ante-natal class, Jack arrives early and Bridget is gratified when Mark shows up as well. Jack and Mark are mistaken for a gay couple with Bridget as their surrogate, much to Mark's discomfort and Jack's amusement. Mark is also jealous at the easy rapport that Jack has with Bridget and his warmth in taking care of her.
Bridget experiences cramps and goes to the hospital, taking Jack with her as she is unable to reach Mark. When at last Mark arrives he is upset to see the two laughing together and embracing. Mark and Jack fight and Bridget sends them out of the room. Outside Jack implies that he and Bridget had sex without condoms making Mark realise that he is less likely to be the father. He leaves once again.
Bridget continues to prepare for the arrival of her baby, now with only Jack by her side. They have a discussion where he asks her to move in with him and confesses that life for him is lonely. Bridget asks him what will happen if Mark is the father and he tells her he will need to re-evaluate their relationship if that is the case. He also confesses that the reason Mark has been absent is because Jack heavily implied that Mark was unlikely to be the father. An upset Bridget goes to talk to Mark, but sees his wife arriving at his house and decides to let him be.
At 9 months, Bridget is fired from her job due to mixing up her guest with his chauffeur. Unemployed, exhausted, and hungry, she goes out shopping but has her card eaten by a cashpoint and locks herself out of the bank lobby, leaving her keys and her food inside. When no one buzzes her up to her building she stands outside in the rain feeling miserable. Mark arrives and breaks in for her. He informs her that he and his wife are going through with their divorce and she was only there to pick up the last of her things. Just as they are about to kiss Bridget's waters break. When his phone rings for work he throws it out the window in a romantic gesture which leaves them without a means to call transport to the hospital. They enlist the help of a local restaurant to get them to hospital but a traffic jam due to protests forces them to walk. Overtaken by contractions Bridget cannot walk and Mark offers to carry her, but almost collapses. Jack arrives just in time and the two of them get Bridget to the hospital.
During labour both Mark and Jack try to help Bridget. However during labour Bridget accidentally punches Jack in the nose. She also reaches to Mark for comfort and he reassures her that she can get through labour and he will love her no matter whether the child is his or not, while sustaining an agonising bite to the wrist by Bridget. While Jack and Mark wait outside, Jack apologies to Mark for his behaviour and Mark accepts. Then Bridget gives birth to a baby boy and all of Bridget's friends and relatives come to visit her in the delivery room. Meanwhile Dr. Rawlings has both men come with her to perform the DNA test. After having bonded for the last 9 months Mark and Jack each wish each other luck.
A year later Bridget prepares to be married. At the altar she is greeted by Jack, holding her son, and then moves forward to marry Mark Darcy. After the wedding Mark and Jack, now friends, head to the reception together, while Bridget carries her and Mark's son, William.
A newspaper lying on a bench reveals that Daniel Cleaver has been found alive in the woods in the wreckage of the plane.
In a post-credit scene. A Christmas photograph shows Bridget, Mark and William wearing a collection of reindeer jumpers worn by Mark in the first film.