On an ordinary Sunday afternoon in Southern California, an uninvited Bert Cousins arrives at Franny Keating's christening party, instigating a kiss with Beverly Keating, the baby's mother. This fateful kiss unravels both their marriages and splices two families into one—a saga that stretches over five decades. The resulting blended family of six children, tied by a collective resentment towards their parents and a bizarre kinship, spends their summers in Virginia, shaping an unbreakable bond. As they grow, their shared narrative takes an unexpected turn when one of the siblings, Franny, embarks on a relationship with a renowned author, Leon Posen, and confides her family's story. This disclosure transforms their childhood memories into a bestselling novel, compelling each member to confront their past, their sense of betrayal, and the strong connection that defines them. 'Commonwealth' delves into a poignant exploration of love, loss, and the intricate patterns of family life, textured with humor, sorrow, and an examination of story ownership.