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 lasting bond that is based on a shared disillusionment with their parents and the strange and genuine affection that grows up between them.