Set in 1914 New Brunswick, Canada, West of Ireland tells the story of the wealthy O'Brien family. As a newlywed, Mrs. O'Brien makes a vow to her older sister-in-law, the formidable Sister Mary-Frances, promising a child to the church. As the years pass and all her children perish, save one, Mary-Kate, Mrs. O'Brien tries to forget her pledge. But Sister Mary-Frances doesn't. When the nun comes to collect her charge, trouble ensues. The O'Briens do everything, short of sacrilege, to keep Mary-Kate out of the clutches of the demanding nun.