Susie Doheny, an Irish Catholic, and Peder Holm, a Norwegian Lutheran, fall in love and marry in South Dakota in the 1890s. Soon their marriage is tested by drought, depression, and family bickering. Susie believes they are being tested by their fathers' God. Peder blames Susie for the timidity of her beliefs; Susie fears Peder's pride and skepticism.

When political antagonism grows between the Norwegian and Irish immigrant communities, it threatens to split their marriage. Against a backdrop of hard times, crisscrossed by Populists, antimonopolists, and schemers, Rølvaag brings the struggle of immigrants into the twentieth century. In Giants in the Earth the Holm family strained to wrest a homestead from the land. In Peder Victorious the American-born children searched for a new national identity, often defying the traditions their parents fought to uphold. In Their Fathers' God, Rølvaag's most soul-searching novel, the first-generation Americans enter a world of ruthless competition in the midst of scarcity.