Diercea begs his father Matusius not to rebel against the law, which requires the annual sacrifice of a noble maiden to Apollo. The name of the victim determines the lot. Only the tsar’s daughters were spared the terrible duty, and that’s because they were sent by the father outside the country. But Matusius believes that he, a subject, in his fatherhood is equal to the king, and in fairness the king should either return his daughters to his homeland and thereby show an example of strict observance of the sacred laws, or free all others from their execution. Dircea believes that the rulers are above the laws, Matusius does not agree with her, he does not want to tremble with fear for his daughter - or let Demophon tremble like the others!

Demophone calls to the palace of his son Timant. He leaves the military camp and hurries to the call. Timant is in a secret marriage with Dircea. If their secret is revealed, Diercea will face death for daring to marry the heir to the throne. Timant enjoys meeting with Dircea and asks her about their son Olint. Dircea says that the boy is like two drops of water like a father. Meanwhile, the deadline for annual sacrifice is approaching. It will soon become known which of the young maidens is doomed to slaughter. The king repeatedly asked the oracle when Apollo would have mercy and cease to demand human sacrifice, but the answer was brief and dark: "The wrath of the gods will subside when the innocent usurper finds out the truth about himself." Dircea is afraid of the upcoming draw. She is not afraid of death, but Apollo demands the blood of an innocent virgin, and if Dircea silently goes to the slaughter, she will anger the god, and if she reveals the secret, she will anger the king. Timant and Dircea decide to confess to the Demofont: after all, the tsar has issued a law, the tsar can also repeal it.

Demophone announces to Timant that he intends to marry him with the Phrygian princess Creus. He sent for her his youngest son Kerint, and the ship should arrive soon. Demofont for a long time could not find a bride worthy of Timant. For the sake of this, he forgot the long-standing hostility of the Thracian and Phrygian kings. Timant expresses bewilderment: why should his wife certainly be royal blood? The demophone insists on the need to honor the covenants of the ancestors. He sends Timant to meet the bride. Left alone, Timant asks the great gods to protect Dircea and protect their marriage.

The Phrygian princess arrives in Thrace. Kerint during the journey managed to fall in love with Creus. Left alone with Creusa, Timant persuades her to abandon the marriage with him. Kreus is offended. She asks Kerint to avenge her and kill Timant. As a reward, she promises him her heart, hand and crown. Seeing that Kerint is turning pale, Creus calls him a coward, she despises a lover who speaks of love, but is not able to stand up for the honor of her beloved with arms in her hands. In the anger of Creus, Kerint seems even more beautiful.

Matusius decides to take Dircea out of Thrace. Dircea suggests that her father found out about her marriage to Timant. She is not able to leave her husband and son. Timant declares to Matusia that he will not let Dircea go, and it turns out that Matusius does not know about their marriage and therefore cannot understand why Timant intervenes in their affairs. Matusius says that Demophonte was angry with him because he, a subject, dared to compare himself with the king, and in punishment for obstinacy ordered to sacrifice Dirzei, without waiting for the lot. Timantus persuades Matusia not to worry: the king is quick-witted, after the first outbreak of anger he will certainly cool down and cancel his order. Guardian Adrast grabs Dirceu. Timant prays to the gods to give him courage and promises Matusius to save Dircea.

Kreusa asks Demofont to let her go home to Phrygia. Demophone thinks that Timant scared off Kreus with his rudeness and incivility, because he grew up among the warriors and was not accustomed to tenderness. But Kreusa says she shouldn't have heard the refusal. Demofont, believing that the princess’s suspiciousness is to blame, promises her that Timant will become her husband today. Kreusa decides: let Timant submit to her father’s will and offer her her hand, and she will amuse her vanity and refuse him. Kreus reminds Demofont: he is a father and a courier, which means he knows what the will of the father and the punishment of the king are.

Timant begs Demofont to spare the daughter of the unfortunate Matusius, but Demofont does not want to listen: he is busy with preparations for the wedding. Timant says he feels an irresistible aversion to Creus. He again begs his father to spare Dircea and admits that he loves her. Demofont promises to save Diercea’s life if Timant obeys his will and marries Creus. Timant replies that he cannot do this. Demophone says: "Tsarevich, so far I have spoken to you as a father, do not force me to remind you that I am the king." Timant equally respects the will of the father and the will of the king, but cannot fulfill it. He understands that he is guilty and deserves punishment.

The demophone complains that everyone offends him: the proud princess, the obstinate subject, the impudent son. Realizing that Timant will not obey him while Dircea is alive, he gives the order to immediately lead Diercea to the slaughter. The common good is more important than the life of an individual: so the gardener cuts a useless branch so that the tree grows better. If he saved it, the tree could die.

Timantus tells Matusia that Demophonus remained deaf to his pleas. Now the only hope for salvation is flight. Matusius must equip the ship, and Timant, meanwhile, will deceive the guards and kidnap Dircea. Matusius admires the nobleness of Timant and marvels at his dissimilarity with his father.

Timant is firm in his determination to flee: a wife and a son are more precious to him than a crown and wealth. But then he sees how Dircea in a white dress and a flower crown is led to the slaughter. Diercea convinces Timant not to try to save her: he still will not help her and will only destroy himself. Timanth is furious. Now he will stop at nothing and nothing, he is ready to betray the palace, temple, priests to the fire and sword.

Diercea prays to the gods to save Timantus’s life. She turns to Creus for intercession. Dircea says that she is innocently sentenced to death, but she asks not for herself, but for Timant, who is threatened with death because of her. Kreus is amazed: on the verge of death, Dircea thinks not of himself, but of Timant. Dircea asks not to ask her anything: when she could tell Creus all her misfortunes, the princess’s heart would burst with pity. Kreusa admires the beauty of Dircea. If daughter Matusia was able to touch even her, then there is nothing strange in the fact that Timant loves her. Kreus struggles to hold back tears. It pains her to think that she is the cause of the suffering of lovers. She asks Kherint to humble Timant’s gays and keep him from reckless actions, and she goes to the Demofont to ask for Diercea. Kerint admires the generosity of Creusa and again tells her of her love. The hope of reciprocity awakens in his heart. It is very difficult for Creus to pretend to be harsh, she is dear to Kerint, but she knows that she must become the wife of the heir to the throne. She regrets that vain pride makes her a slave and makes her suppress her feelings.

Timant and his friends seize the temple of Apollo, overthrow the altars, extinguish the sacrificial fire. Demophonus appears, Timant does not admit him to Dirzee. Demophone orders the guards not to touch Timant, he wants to see what filial impudence can go to. Demophone throws a weapon. Timant can kill him and offer his unworthy beloved hand, still smoking with the blood of his father. Timant falls to the feet of Demofont and gives him his sword. His crime is great, and he has no forgiveness. Demophone feels his heart tremble, but takes control of himself and orders the guards to chain Timant. Timant submissively hands up. Demophone orders to kill Dircea right now, in his presence. Timant cannot save her beloved, but asks her father to have mercy on her. He reveals to Demophont that Dircea cannot be sacrificed to Apollo, for God requires the blood of an innocent virgin, and Dircea is a wife and mother. Sacrifice is delayed: another victim must be found. Dircea and Timant are trying to save each other, everyone is ready to take all the blame upon themselves. Demophone orders to separate the spouses, but they ask permission to be together at the last hour. Demofont promises that they will die together. Spouses say goodbye.

Guardian Adrast sends Timanthe last request to Dircea: she wants Timant to marry Creus after her death. Timant refuses angrily: he will not live without Dircea. Kerint appears. He brings the good news: Demofont relented, he returns to Timant his fatherly love, wife, son, freedom, life, and all this happened thanks to the intercession of Creusa! Kerint tells how he led Demofeus to Dirtsey and Olint, and the king embraced the boy with tears in his eyes. Timant advises Kerint to offer a hand to Creusa, then Demofont will not have to blush for breaking the word given to the Phrygian king. Kerint replies that he loves Creusa, but does not hope to become her husband, for she will give her hand only to the heir to the throne. Timanth has renounced his heir rights. He owes his life to Kerint and, giving way to the throne, gives only part of what he owes.

At this time, Matusius learns that Dircea is not his daughter, but Timant's sister. Before his death, Matusia's wife handed her husband a letter and made him swear that he would only read it if Diercea was in danger. When Matusius was preparing to flee, he remembered the letter and read it. It was written in the hand of the late queen, who certified that Dircea was the royal daughter. The tsarina wrote that in the palace temple, in a place where there is no access to anyone except the tsar, another letter is hidden: it explains the reason why Dircea was in the house of Matusius. Matusius expects Timant to be delighted, and does not understand why he is turning pale and trembling ... Left alone, Timant surrenders to despair: he goes out, he married his own sister. Now it is clear to him that the anger of the gods has drawn upon him. He regrets that Creusa saved him from death.

Demophone comes to hug Timant. He pulls away, ashamed to raise his eyes to his father. Timantha wants to see Olint, drives away Dircea. He wants to retire to the desert and asks everyone to forget about him. The demophone is anxious, he is afraid that his son is not damaged in his mind.

Kerint convinces Timant that he is not guilty of anything, because his crime is involuntary. Timant says he wants to die. Matusius appears and announces to Timant that he is his father. Dircea reports that she is not his sister. Timant thinks that in order to console, they deceive him. The demophone says that when the daughter of the queen was born, and the wife of Matusius - the son, the mothers exchanged children so that the throne had an heir. When Kerint was born, the queen realized that she had stripped the throne of her own son. Seeing how Demophone loves Timant, she did not dare to reveal a secret to him, but before her death she wrote two letters, one she gave to her confidante - his wife Matusia, and the other she hid in the temple. The demophone tells Creus that he promised her husband and heir to the throne to her husband and is now happy that he can keep his word without resorting to cruelty: Kerint is his son and heir to the throne. Kreus accepts Kerint's offer. Kerint asks the princess if she loves him. Kreus asks her consent to be considered the answer. Here only Timant understands that he is that innocent usurper that the oracle was talking about. Finally, the Thracians are spared the annual sacrifice. Timant falls to the king's feet. Demophone says that he still loves him. Until now, they loved each other in debt, from now on they will love each other by choice, and this love is even stronger.

The choir sings that joy is stronger when it comes to the heart, dejected by misfortune. But is the world perfect, where in order to fully enjoy it, one must go through suffering?