When the atheistic ranting of Irish-American author James Mulcahy upsets the inhabitants of the Irish village to which he has retired, a mob threatens him. But moments after he has dared God to strike him dead, a stranger appears and does so. The man, dubbed "Johnny Nobody" by the press, claims no knowledge of Mulcahy or even of himself. He asks the help of the village priest, Father Carey, in his upcoming trial for Mulcahy's murder. While the amnesiac Johnny goes to trial, Father Carey mulls questions of belief raised by the case.