Mild-mannered nineteen-year-old cab driver Ed Kennedy finds himself a hero after an impulsive citizen's arrest. Ed captures a hapless bank robber during a failed heist and the local media publicizes his heroics, making his face and name well known. His alcoholic father has just died, leaving behind his bitter wife Bev. Ed and his three adult siblings have moved out and on with their lives. Ed’s older sisters, Katherine and Leigh, are married and have children. His brother Tommy—younger, faster, and smarter than Ed—attends university in the city and dates a woman who could pass as a supermodel. Ed lives with his dog Doorman in a small house he rents from his boss at Vacant Taxis. At nineteen, Ed takes stock of his life. He has no prospects for improving his financial outlook, having neither skills nor interest in higher education. The love of Ed’s life, Audrey, has set strictly platonic terms on their friendship while she pursues romance with others. His mother Bev treats him with disdain. Ed’s social life consists of playing a card game called Annoyance with his three friends several times a week. 

Everything in Ed’s life changes when Ed begins receiving anonymous mailings of ace playing cards with clues written on them. He resists his initial impulse to throw the first card away. Something about it calls to him. The cards arrive over six months bringing a sense of purpose to Ed’s mundane existence. He unravels the clues, investigates the circumstances, assesses the needs, and intervenes in people’s lives. As a result of the three clues on the first card, the ace of diamonds, a brawny husband who comes home from his nightly drinking and brutally rapes his wife is persuaded at gunpoint to disappear. Eighty-two-year-old Milla Johnson, who still awaits the return of her beloved husband Jimmy (whom she has forgotten died in World War II), finds that Jimmy has returned in the person of Ed. A talented fifteen-year-old runner named Sophie who lacks self-esteem receives encouragement from Ed to be herself. With the ace of clubs, Father O’Reilly’s dwindling congregation gets a whole new flock of congregants through Ed’s Meet a Priest Day. Angie Carusso, a single mother of three, is comforted to learn that someone sees her sacrifice. Gavin Rose’s war with his brother Daniel ends when they bond over a common enemy. 

With the ace of spades, the sender sets missions for Ed that have personal significance. The Tatupu family reminds Ed of the goodness he loved in his father. The discovery of Ed’s mother Bev’s marital infidelity reframes his father’s alcoholism, allows Bev to unburden herself of guilt, and enables Ed to view his mother as an individual. Audrey sees Ed in a new light when she learns of Ed’s activism to help people better their lives. The final ace, the ace of hearts, challenges Ed’s status quo with his three friends, Ritchie, Marv, and Audrey. He risks ending his precious friendships to be a force for needed change in their lives. He confronts Ritchie about the emptiness of his life. He reunites Marv with the child he fathered and the woman he loves. And he finds a way to express his passionate love for Audrey in a way that she returns. 

Ed receives messengers that guide him, motivate him out of his reluctance, and punish his mistakes. He comes to understand that these people are also recipients of aces and they are all part of a network of messengers. They are not supernatural go-betweens. Ed rejects the title of “saint,” a religious term for a person who does the will of God, and he insists he’s a fallible human. For example, Ed uses violence to achieve certain ends. He exploits people’s weaknesses to achieve his goals. He suffers from self-doubt and anxiety and often hesitates for too long. But Ed perseveres and changes throughout the book from a passive observer waiting for life to begin to an active agent for good in his world. 

Just when Ed relaxes that he has passed the test, the joker card arrives with his address. The menace contained in that simple message causes Ed to withdraw from his world. Afraid of the choices he might need to make in his own life, he avoids all social contact. The sender finally reveals himself to explain everything. Ed discovers the sender in Ed’s house, making himself at home, sitting on Ed’s couch and petting his dog. The sender has the small frame and youthful appearance of a boy but the demeanor of a man. Without giving his name, he tells Ed he came to town a year earlier. He claims responsibility for the key events in Ed’s life: the death of his father, his job as a cab driver, the bank robbery, and the messengers. The claims the sender makes strain credulity and challenge Ed’s belief in himself. Reality as Ed knows it is at stake. In the end, Ed finally decides that what he accomplished in his community and the person he has become are more real than the sender’s claims.