While King Endre II of Hungary is fighting abroad, Queen Gertrud plays hostess to leading members of the Court. Prince Ottó, the queen’s younger brother, is trying to seduce Melinda while her husband Bánk bán, the king’s deputy, tours the country. Petur bán, the leader of the restive Hungarian noblemen, looks on the festivities with disdain. He tells his companions that he has secretly sent a messenger after Bánk to ask him to come back and see, with his own eyes, the double threat looming over the country and his own wife, Melinda.
Melinda reacts coldly to Ottó's public courtship and is appalled to realise that Queen Gertrud herself is willing to support the liaison. Arriving home, Bánk watches as Ottó kneels before Melinda but he leaves before he can witness his wife's firm rejection of the prince. Melinda provokes an angry response from Gertrud. Believing Bánk to be far away, the malcontents swear to avenge him.
Bánk resolves to approach Gertrud and reveal the country's problems to her. He is discovered by Tiborc, an elderly peasant who once saved Bánk's life in a long-ago battle. Soon, Melinda herself appears, distraught and on the brink of madness having been raped by Ottó. Bánk curses their child, but Melinda's pleading and the memory of her old beauty and purity compel him to forgive her. He then asks Tiborc to accompany his wife and their child to his castle on the bank of the Tisza.
In spite of the late hour, Bánk heads for Gertrud's chambers. The queen demands an explanation for his sudden return, while he calls her to account for not even attempting to remedy the country's wounds. After she threatens him with death, Bánk also holds her responsible for Melinda's lost honour. Gertrud curses Bánk, Melinda and Ottó. The prince enters upon hearing his sister's cries for help, but flees at the sight of Bánk. Gertrud draws a dagger, only to be overpowered by Bánk.
A storm overtakes Tiborc, Melinda and her child on the banks of the Tisza. Melinda’s own tragedy returns to her in a series of deranged hallucinations and she casts herself into the raging river with her little son in her arms.
In the palace, Endre II and his court are gathered around Gertrud's catafalque. A royal officer reports that he has killed Petur, the queen's murderer. Bánk enters and tosses his chain of office, the badge of his authority, onto Gertrud's coffin. He admits to the murder, but will not accept the king's judgement over him. His punishment is delivered not by the king, but by the story told by Tiborc, who has arrived with the bodies of Melinda and their son. Having lost everything Bánk breaks down and ends his own life.