The story follows Hayley, a dedicated young nurse who starts working in a home for the elderly. There she meets Keith, a sweet and fatherly old man who lives in the home. Keith reminds her of the father she never had and gradually the two form a strong bond. Every day Keith believes that his beloved only son James will be coming to take him home and he waits for James on the porch, watching the road for his arrival. He passes the time looking at James' pictures and talking of him proudly to Hayley but James never shows up. Puzzled, Hayley looks into Keith's history and discovers the truth: that James had sold his father's house and then abandoned him ruthlessly at the doors of the nursing home. Enraged, Hayley believes James to be unworthy of Keith's unconditional love. As the story progresses, we gradually discover that Hayley had been keeping a secret of her own: thinking the worst of James and believing she is protecting Keith out of love, she has been hiding James' letters. But as Keith's love only grows stronger, Hayley is forced to look into herself and decide what exactly true love really means.
Prodigal is based on the Parable of the Prodigal son, in which a rich man's younger son asks for his inheritance in advance and leaves the family home. Put in context these actions, like James' in the film, are effectively an abandonment and estrangement between father and son. Despite this insult and rejection, the father in the parable never wavers in his love and this is seen in how he waits constantly by the gate for his son to return. This love and faith is mirrored in Keith, who believes his son will repent and come back for him and is so eager for the moment of reunion that he waits at the porch. Amidst all this is the figure of the elder son, embodied in the film's protagonist Hayley. Like the jealous elder son who cannot comprehend the father's actions and begrudges his love, Hayley judges James and wishes to keep him away, ostensibly to protect Keith but also to keep Keith's love to herself. At the end of the parable the eldest son is taught an important lesson in love by the father and this is reflected in Hayley's journey of discovering the complexities of true, unconditional love.