Act 1
The 4th century AD. Valens, the Roman governor of Antioch, issues a decree that in honour of Diocletian's birthday all citizens will offer sacrifice to Venus, the Roman goddess of love, and Flora, a fertility goddess of the spring, on pain of death, and puts Septimius in charge of enforcing this.

Didymus, a soldier secretly converted to Christianity, asks that citizens whose consciences prevent them making sacrifices to idols be spared punishment, which Valens dismisses. Septimius suspects Didymus is a Christian and affirms his own loyalty to the law although he pities those who will be condemned to die by the decree and wishes he could be allowed to extend mercy to them.

Theodora, a nobly-born Christian and her friend Irene are worshipping with their fellow believers in private rather than joining in the festival for the emperor's birthday when a messenger brings news of Valens' decree. Septimius comes to arrest them - Theodora expects to be put to death but is informed that instead she has been sentenced to serve as a prostitute in the temple of Venus. Theodora would much have preferred to die, but is led away to the temple. Irene informs Didymus who goes in the hope of either rescuing her or dying with her. The first Act closes with a chorus of Christians praying for the mission's success.

Act 2
At the start of the second Act the festival in honour of the emperor and the goddesses is being enjoyed by the pagans. Valens sends Septimius to tell Theodora that if she doesn't join in with the festival by the end of the day, he will send his guards to rape her. The crowd expresses their satisfaction at this sentence. In the temple of Venus which serves as a brothel, Theodora is frightened, but her mood changes as she contemplates the afterlife. Didymus confesses to his friend and superior officer Septimius that he is a Christian and appeals to the other man's sense of decency. Septimius allows Didymus to visit Theodora. At first Theodora appeals to Didymus to kill her and put an end to her suffering, but instead Didymus persuades her to conceal her identity by putting on his helmet and his uniform and escaping, leaving Didymus in her place. Back at their hideout, Irene and the Christians recall the miracle of The Widow of Nain and hope that, should the lovers die, they will find a new life in heaven.

Act 3
As the third part opens the Christians celebrate Theodora's safe return. However she feels guilty that she endangered Didymus's life in order to save her own. A messenger informs them Didymus has been captured and that Valens has changed Theodora's punishment to death. Theodora goes to offer herself in Didymus' place, despite the protests of her faithful friend Irene. As Valens sentences Didymus to be executed, Theodora enters demanding that she die and Didymus be saved. Both Didymus and Theodora argue that they should die in place of the other. Septimius is moved by this, and pleads for clemency. Valens, however, condemns both Didymus and Theodora to death and they sing a duet to their immortality.