The action is set in Prusa (modern-day Bursa or Brussa), the capital of Bithynia, in 1403.
The Ottoman sultan Bajazet has been defeated by the Tatar emperor Tamerlano, and is now held prisoner in his own palace. The Greek prince Andronico, an ally of the victor, succeeds in securing the freedom of Bajazet, who, however, would rather kill himself than be humiliated before Tamerlano. He only chooses to remain alive because he knows that his gesture would endanger his beloved daughter, Asteria, who loves Andronico and is loved by him. Tamerlano, however, has also fallen in love with her, despite being promised in marriage to Irene, the princess of Trebizond. The emperor, who is unaware of the love of the two young people, decides to give Irene in marriage to Andronico, if he helps him to win Asteria. He then tells Asteria
that, if she consents to marry him, she will save her father’s life, while Andronico, who is to ask for Bajazet’s agreement to the marriage, will have Irene. Asteria is confused: she thinks that Andronico has betrayed her and so she decides to accept Tamerlano’s proposal in order to save her father. Bajazet, on the other hand, refuses to consent to Asteria’s marriage to Tamerlano and advises Andronico to reassure Asteria of his love for her. Andronico hesitates in the fear of harming Bajazet, and Asteria is cold and detached towards him. When Irene arrives in Prusa, she is not admitted to the presence of Tamerlano. Andronico informs her of the situation, promises to help her and advises her to come to court in the guise of an ambassador with the help of Leone, a friend of both Andronico and Tamerlano.
Tamerlano tells a stunned Andronico that Asteria has agreed to marry him and that the wedding will take place even without Bajazet’s consent. Asteria and Andronico grow increasingly more mistrustful towards each other. The Greek prince is however set on preventing the marriage and he informs Bajazet of the new turn of events, while Tamerlano, with Asteria by his side, receives Irene disguised as an ambassador. Irene chastises Asteria for unduly taking the place of the princess of Trebizond alongside Tamerlano, who states that he will only marry Irene should Asteria cease to attract him. Mysteriously, Asteria assures Irene of her intention to make herself disagreeable to the emperor. Meanwhile, Andronico is in despair and threatens to kill himself if Asteria marries the emperor, but Bajazet is determined to stop his daughter. In the throne room, Tamerlano announces his engagement to Asteria just as Bajazet enters. Instead of bowing before his enemy, he throws himself to the ground in an attempt to stop his daughter from sitting on the throne next to Tamerlano. When her father reminds her of her oath to avenge him, Asteria confesses in front of all those assembled that she has only accepted Tamerlano’s proposal in order to be able to kill him on their first night together. The furious Tamerlano threatens to have her and her father put to death, while Asteria reconciles herself with Bajazet and Andronico.
Bajazet and Asteria are imprisoned and they contemplate suicide by poison should the situation worsen. But Tamerlano relents on his decision and orders Andronico to renew his marriage proposal to Asteria. At first, Andronico is undecided about what to do, but he eventually declares his love for Asteria, who reaffirms her hatred for Tamerlano. As a punishment, Tamerlano
orders that Bajazet be beheaded and that Asteria be given in marriage to a slave. When the girl kneels begging for her father’s life, Bajazet enters and scolds his daughter for prostrating herself before the enemy. Tamerlano then commands the presence of both father and daughter at the banquet that is being prepared. Asteria and Andronico bid each other farewell. Leone
urges Irene to take her rightful place on the throne next to the emperor. During the banquet, Asteria attempts to kill Tamerlano by poisoning his cup. Irene, however, stops the emperor from drinking it, thus saving his life, and then reveals her true identity. Reconciled with Irene, the emperor orders Asteria to give the cup to her father or to her lover. The girl tries to drink from
it herself, but Andronico stops her. When Tamerlano orders Asteria to be given to the slaves, Bajazet decides to escape the emperor’s tyranny by poisoning himself. He curses Tamerlano and swears to torment him from hell. Asteria wants her father to kill her before he dies, but Bajazet says she could never perform such an act. Asteria and Andronico both intend to kill themselves, but the emperor now seems to have calmed down. He will marry Irene and leave Asteria and the throne of Byzantium to Andronico. After so much pain and suffering, the future appears less dismal.