Plot
Alice and Paul are struggling siblings living separate lives apart from their family. Paul, a gay rehab center worker, is in a relationship with his live-in partner Dominic. Paul becomes neurotic when Dominic hints at opening their relationship to other partners. Alice works as an executive assistant at a modern architecture firm and is having an affair with Jonathan, her married boss. Her attempts to convince him to leave his wife are met with constant excuses. She later finds out that his wife has recently had a baby, leaving her hopes of marrying him further away than ever.
Both siblings are estranged from their mother Donna, and their older half-sister Eloise, a product of Donna's first marriage to Henrique, a wealthy French-British playboy she met in the UK decades before. Donna ends the marriage after she discovers that Henrique has been unfaithful. She later marries a man named Bill and has a seemingly happy marriage with him that produces Alice and Paul. Eloise spends the summers with her father in Britain and never really connects with her half-siblings.
Years pass, and Eloise, now wealthy herself through her father's influence, becomes engaged to a man named Ollie. She attempts to bridge the gap with her family in the States by inviting them to her wedding. While a now-widowed Donna enthusiastically accepts, Paul and Alice, resentful of their mother's tie with her first husband, reluctantly accept. However, Eloise's well-meaning gesture of paying for Alice's motel room upsets Alice and causes further resentment.
After Paul loses his job for empathizing with a patient, Dominic suggests that they save money by staying with Alcott, Dominic's old college professor, who is also gay. Paul tries to maintain an open mind when Dominic and Goulding engage in sexual activity and attempt to draw him into it, but then he reacts negatively and leaves in a huff.
Meanwhile, Donna reconnects with Henrique and has sex with him. He hints at wishing to reconcile with her, which delights Eloise but infuriates Paul and Alice.
Through all this is a subplot involving Alice and Dennis Bottoms, a Nebraskan whom she meets on the plane and with whom she later has a sexual encounter at the hotel. She begins to spend a lot of time with him and enjoys his company, but then receives a text from Jonathan telling him he is on his way to London. She falsely confesses to Dennis that she is in a relationship, but is ending it. After receiving another text from Jonathan saying that he is outside of the venue for the rehearsal dinner, she is horrified to discover that it's Jonathan's wife, who was sending the messages from his phone after becoming suspicious of Jonathan's infidelity. She engages in a physical confrontation with Alice, and Dennis tells Alice he no longer wants anything to do with her. A drunken Paul also shows up at the dinner, having just broken up with Dominic, and discovers Henrique kissing a much younger woman. Paul punches Henrique and then urinates on his shoes.
Fed up with her siblings' drama, Eloise throws all three out and tells them they are unwelcome at the wedding, but not before lashing out at Paul over his anger with his mother after removing of all traces of Bill's existence after his death, yet keeping a picture of Henrique. Paul discovers from Eloise that while Bill outwardly accepted Paul's homosexuality after coming out, he secretly hated it. Eloise also tells Paul that Donna threatened to divorce Bill if Paul ever were to discover Bill's homophobia, likely leading to the cooling of their relationship in Bill's final years. Realizing that Donna all this time protected him from the man who grew to hate him, but whom he worshipped, Paul weeps bitterly and makes up with his mother, as does Alice.
While Dennis is upset at Alice's deception, he agrees to drive them all back to the hotel after the trio are booted from the dinner and jailed.
The next morning, as they prepare to fly back home, they are met by a frantic Ollie, who says that Eloise has disappeared and is nowhere to be found. On a hunch, Donna finds her at a Taco Bell restaurant (a childhood indulgence allowed by Donna but forbidden by Henrique) gorging on fast food. Alice persuades Donna to let her talk to Eloise alone. Alice confesses she held a grudge because Eloise did not come to her aid after she suffered a miscarriage. Eloise apologizes and confesses that she stayed away because shortly before that news arrived, she learned that she can never conceive—a secret she has yet to share with Ollie—and was not emotionally available to support her sister. Alice accepts this and bonds with her big sister at last and also persuades her to be honest with Ollie.
A year passes and the siblings are together again with Donna, this time much more happily. Ollie and Eloise are still a couple, having adopted a baby, and Dennis has forgiven Alice and they are back together.
