Ally is the showrunner of Dessert Island, a successful but waning reality TV series. After filming the season's final episode, she is informed that the show will be cancelled. With no social life outside of work, she decides to take a break and visit her mother in Leavenworth, Washington.
At a bar in her hometown, she runs into an ex-boyfriend, Sean. They spend the night drinking together, and kiss in the car when Sean takes her home. He refuses her invitation for sex, and she leaves awkwardly. Later that day, she visits Sean's house intending to apologize for the incident, but learns that he is getting married that weekend and his family is throwing a party. She tries to leave, but Sean's mother Jojo insists she attend the wedding to film it.
Ally learns that Sean's fiancée, Cassidy, is alienated from her parents. She feels suspicious about her, and gets her assistant, Kayla, to dig up information about Cassidy's family. Meanwhile, a streaming platform is interested to renew Dessert Island for another season and wants to meet with Ally.
Cassidy is uneasy about Ally's presence at wedding parties. She performs with her punk rock band at a bar, and later shares a table with Ally to talk about Sean. Cassidy tells Ally that she plans to stop touring with her band to settle down with Sean after marriage. Ally notices that she is unhappy about this.
Kayla finds information about Cassidy's parents, and Ally asks her to send them an anonymous invitation to the wedding. When Ally and Cassidy spend more time with each other, Ally realizes her impression of her was mistaken. They bond over common interests, even ending up streaking together on a golf course after smoking cannabis. Sean is distracted by their growing closeness.
On the eve of the wedding, Cassidy is surprised to see her parents. Ally does not confess to inviting them, leading Cassidy to blame Sean for it and call off the wedding. At their hotel, Ally tries to correct her mistake by getting Sean and Cassidy to understand each other better. Sean realizes he is suppressing Cassidy's career and apologizes. Meanwhile, Ally takes the blame for inviting Cassidy's parents, and Cassidy reneges on cancelling the wedding.
Ally leaves the event permanently and returns to her mother's house, where they have a heart-to-heart about being one's true self. Ally leaves for Los Angeles, but makes a pact to visit more often. Dessert Island is picked up for another season, but Ally passes showrunning duties on to Kayla. She chooses to focus on her interests, picking up another show instead, which centers around the topic of nudism.
On the review aggregator website Rotten Tomatoes, 72% of 86 critics' reviews are positive, with an average rating of 6.1/10. The website's consensus reads: "While it might have been more satisfying with a sharper focus, Somebody I Used to Know is a funny, uniquely nuanced rom-com with some interesting things to say."[8]Metacritic, which uses a weighted average, assigned the film a score of 57 out of 100, based on 18 critics, indicating "mixed or average reviews".[9]
