1213 Queen Gertrude of Meran's compatriots are playing the dorba in the royal castle. The unpeaceful Hungarian nobles are watching the dárido, and under the leadership of King Petur they are plotting. Their anger is only increased by the fact that while the king's viceroy, Bánk bán, who is fighting in the distance, is roaming the country, his wife, the beautiful Melinda, who has been left at court, is being besieged more and more openly by the queen's lecherous brother Otto. Petur secretly recalls Bánko, who, however, accuses the unpeaceful of treason. But Otto's confidant, the knight Biberach, tells Banach that the prince is trying to seduce Melinda. Otto, encouraged by the Queen, prepares to make a decisive attack. He pursues his lover Melinda in the castle's cloisters and chapel, and she rejects him contemptuously, but is nevertheless in a misunderstood position. Bánk, who has just seen them, is shocked: it seems that his adoring spouse is indeed unfaithful...