Ben (Bruce Willis), a writer and Katie (Michelle Pfeiffer), a crossword puzzle editor are a married couple with two children who have concluded, after 15 years of marriage, they no longer love each other. They take the period while their kids are at summer camp to try a separation, during which discuss they discuss their situation with their respective friends, and reflect (through flashbacks) on their original meeting, the highs and lows of their married life, and their attempts to reconcile their relationship with various therapists. During this period, Ben tries to write a history of his Grandparents long and happy marriage, and Katie begins a friendship with her divorced Marty, who wedding has ended.
During Parents Weekend at camp, they attempt, with limited success to put on a happy face while their kids are around. Their evening at a local hotel leads to a remembrance of their failed last attempt to rekindle their romantic feelings, and on the drive home they agree to seek an amicable divorce.
After Ben moves out, he and Katie argue with mutual friends about the cause of their relationship ending; Ben is upset that Katie can never be spontaneous; Katie is frustrated by Bens lack of planning and preparedness. Ben attempts to reconcile, but discovers Katie cooking dinner with Marty in their house.
Ben & Katie agree to tell their children about their planned divorce after picking them up from camp. During the drive, they both reflect on the totality of their relationship, both the happy and angry moments. After picking the kids up, they realize that, despite all the anguish, they still love each other, and commit to living mostly happily ever after.