One Sunday afternoon in Southern California, Bert Cousins shows up at Franny Keating's christening party uninvited. Before evening falls, he has kissed Franny’s mother, Beverly—thus setting in motion the dissolution of their marriages and the joining of two families. Spanning five decades, Commonwealth explores how this chance encounter reverberates through the lives of the four parents and six children involved. Spending summers together in Virginia, the Keating and Cousins children forge a last bond that is based on a shared disillusionment with their parents and the strong and genuine affection that grows up between them. When in her twenties, Franny begins an affair with the legendary author, Leon Posen and tells him about her family, the story of her siblings is no longer hers to control. Their childhood becomes the basis for his wildly successful book, ultimately forcing them to come to terms with their losses, their guilt, and the deeply loyal connection they feel for one another.