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.

Edina frantically organises her cluttered house in order to host old friends Bettina and Max, who were once obsessed with minimalism. Feeling ignored by Edina, Patsy leaves in search of another lunch partner. When Bettina and Max arrive, Edina is disappointed to learn that they are now the hysterical, neurotic parents of a newborn baby. As the couple wreak havoc, Edina quickly leaves the house. At a restaurant, Edina and Patsy compete against each other by attempting to befriend famous people: Patsy invites herself to lunch with Britt Ekland and Zandra Rhodes, while Edina forces her client Lulu to have lunch with her. Both lunches end awkwardly. Back home, while Saffron and Mother comfort Bettina, they hear Edina and Max having sex over the baby monitor. After the couple finally leave the intended result of Edina's fling she and Patsy make peace.