The war between Rome and the Parthians seems decided: the Roman general Lucio Vero has defeated the Parthians; their king, Vologeso, has supposedly fallen in battle and his consort Berenice has been brought, along with other prisoners, to Lucio Vero's court at Ephesus. Lucio Vero is in love with Berenice, but she remains faithful to Vologeso beyond death.

During a banquet, a Parthian slave plans to poison Lucio Vero. Berenice, unawares, is about to drink the poisoned wine, when the Parthian reveals his murderous intent and so saves Berenice's life. She alone has recognised in him Vologeso, believed dead. Lucio Vero has him thrown into prison.

Lucio Vero receives a visit from Rome: the Emperor's daughter Lucilla, his betrothed, as well as Flavio, ambassador of the Senate. Both urge Lucio to return to Rome, Flavio from reasons of State, Lucilla from love.

Vologeso, unarmed, is to be thrown to the lions in the arena. He sees Berenice sitting at Lucio Vero's side and accuses her of infidelity. But Berenice want to die with Vologeso, and hurls herself into the arena. Lucio Vero's sword saves Berenice's life, but Vologeso's too. Lucio Vero has understood: the Parthian is King Vologeso. Lucilla has also realised that Lucio Vero loves Berenice. Berenice and Vologeso hope for clemency by the Roman general. Lucilla wants clarification: Lucio Vero is silent.

Lucio Vero discusses with his servant Aniceto how he could win Berenice. He offers Vologeso his freedom and his kingdom if he will give up Berenice: Vologeso furiously refuses. Berenice wants to save Vologeso's life; but how can she do so without being unfaithful to him? Lucio Vero presents her with the choice: either her hand or Vologeso's head. She promises him her heart. Lucio Vero triumphs, but yet doubts assail him: will Berenice really love him?

Lucilla hears from Lucio's lips how his heart is bestowed: he loves Berenice. She and Flavio are to go back to Rome: he suggests also that she has certainly, some time ago, had another lover. Lucilla is incensed, for her love is unshakeable.

Lucio Vero informs Vologeso that he has lost Berenice. Berenice corrects him: she has promised Lucio her heart but he needs to tear it from her breast. Both are ready to die for their love. Lucio rages and puts her in chains: such great love bewilders him.

The Roman Army is no longer prepared to obey a general who insults the daughter of the Emperor and loves the consort of an enemy. The soldiers, with Flavio at their head, are ready to rebel. Flavio frees Vologeso from his cell: he is to return with Berenice to his kingdom, for Berenice must vanish from Lucio's life.

Lucio Vero grasps at a last means: so as to win Berenice he stages, with Aniceto, a macabre performance, at the height of which Berenice will be presented with a covered object, supposedly Vologeso's head. But Berenice will not give in to Lucio's wooing: now she wishes to be united in death with Vologeso. Lucio Vero is beside himself when the rebels, with Vologeso and Flavio at their head, storm the palace. Lucio Vero has the choice: he must either marry Lucilla and become the Roman Emperor, or he must die. He chooses life. He restores their freedom to Vologeso and Berenice and begs Lucilla's forgiveness.