Assumption is set mainly in a small prairie town in Saskatchewan and on an isolated lighthouse island off the west coast of British Columbia. For over five decades, the lives of Laura, Michael and their mother Elizabeth are affected by the challenges of the 1930s Depression, World War II and the major political, economic and societal changes following the war. They are also challenged by secrets and mysteries that often cause them to make faulty assumptions, which affect their life choices and relationships.
When Elizabeth dies, Laura and Michael discover her diary, which had been hidden for thirty-five years. Will the contents of the diary be the key that unlocks the secret that has overshadowed most of their lives?