When Bert Cousins shows up at Franny Keating's christening party uninvited and later kisses Franny's mother, he sets in motion the joining of two families. Commonwealth weaves across 50 years to tell how that chance encounter changed the lives of the six children and four parents involved. When in her twenties, Franny meets the famous author Leon Posen and tells him about her family, the story ceases to be anything she can control. Told with equal measures of humour and heartbreak, Commonwealth is a meditation on inspiration, interpretation and the ownership of stories, as well as the far-reaching bonds of love and responsibility.