The Holy Nail introduces Nick Genova and Joe Cohen, two American soldiers who reach Milan one day ahead of Allied forces at the end of World War Two. The next day, with the city in chaotic celebration, they enter the battered Milan Duomo with a Partisan woman and together they steal a famous Catholic relic, the Holy Nail, said to be a remnant of the Crucifixion. It has been the soul of the cathedral for fourteen hundred years. Complications arise immediately as the two men try to negotiate a ransom from the Catholic Church. The Church authorities, aided by the Milan police and an international investigator, pursue a number of leads, but they are unaware of Mafia interest in the Nail. An unholy interplay of characters and motives ensues.