A woman is led to the edge of madness as she wrestles between her spiritual life and her romantic longings in this unusual drama. Martha Hayes (Genevieve Bujold) is a young woman who was raised in a small rural community. A deeply committed Christian, Hayes moves to Montreal, where she earns a meager wage working with the choir at an Anglican Church.
Hayes has also been asked to look after a child with health problems, and when the child unexpectedly dies, she falls into a deep depression. While in conference with Father Michael Ferrier (Donald Sutherland), a priest who has come to her church as part of a interfaith music festival, Hayes breaks down and confesses her love for him.
Ferrier responds by leaving the church to marry her; however, Ferrier is not well suited for life in the secular world, and their marriage soon collapses, leading Hayes to desperate measures. Genevieve Bujold's performance in Act Of The Heart earned her the Best Actress award at the 1970 Canadian Film Awards.