The play, which is based on the life of the historical Dorothea of Caesarea, is set in Caesarea during the reign of the Roman emperor Diocletian, one of the last of the pagan emperors before Constantine I's conversion to Christianity. The opening scene shows the arrival in the city of Diocletian and his daughter Artemia, and introduces the local governor, Sapritius, and his main persecutor of Christians, Theophilus. Theophilus is ruthless and brutal in his pursuit of Christians to torture and execute; he is assisted by his secretary Harpax, who is an actual devil in human guise, and who uses second sight to aid his master's activities. The Roman emperor and his court are celebrating a victory over rebellious vassals; Diocletian gives his daughter a choice among three captured kings for her husband — but Artemia prefers Antoninus, the son of governor Sapritius and the hero of the recent battle.