Edina is none too pleased about her 40th birthday. As opposed to the glamorous, star-studded surprise party she had hoped for, Edina is disappointed to learn that Saffron has planned a quiet lunch with family and friends—including both of Edina's ex-husbands, Justin and Marshall, and their respective new lovers, Oliver and Bo, as well as Patsy and Mother. Edina behaves erratically during the party, switching between self-pity and spite, and she soon provokes Oliver, her least favourite guest, into making an angry exit. Justin and Marshall are shocked to discover that they have both been paying alimony to Edina. While Edina and Patsy sing drunkenly on a new karaoke machine, Justin and Bo secretly plot to cut off Edina's alimony.

Patsy becomes the subject of a tabloid sex scandal following an affair with a married MP. Edina checks into a hospital for surgery after injuring her big toe. Patsy tags along, planning to undergo a chemical peel in preparation for her interview with Hello! magazine. While anesthetised, Edina dreams that she is on her deathbed, surrounded by idealised versions of family and friends, while Patsy envisions herself looking young for her Hello! spread. Edina awakens to find that her pains were caused by a stray acupuncture needle that worked its way from her cranium to her toe. Meanwhile, a botched facelift leaves Patsy's face covered in burns. The Hello! spread is also a disaster, with the article reporting Patsy's age as 49, blaming her for the sex scandal and revealing her horrific facelift.

Edina is left feeling helpless after Saffron leaves home to attend university. Patsy loses her job when Ella magazine folds; she later receives an offer to work with her colleague Magda at a New York magazine. Edina and Patsy commiserate drunkenly with each other over old age and the sudden changes in their lives. Saffron returns for a visit, appalled at their behaviour. When Edina announces plans to sell the house before embarking on a journey of self-discovery, a livid Saffron storms off. Edina's plans also mean that she is cutting off Patsy. Embittered, Patsy leaves as she decides to take the job in New York.

Edina joins a New Age commune, but she struggles to fit in and quickly becomes bored. Patsy finds herself disillusioned with her new job in New York due to creative differences with her co-workers. Saffron settles into university life and Mother spends her days at the Monsoon house, drinking champagne and ordering items from the shopping channel. After realising that their lives are not the same without each other, Edina and Patsy happily reunite on top of Carnegie Hall Tower before returning to London. Back home, Mother has been arrested for squatting and credit card fraud. A flashforward to 25 years into the future provides a glimpse into the lives of Edina, Patsy, Saffron and Mother.