Barbara Radziwillówna, the daughter of the noble house of Radziwill, accidentally meets the future Polish king Zygmunt August under embarrassing circumstances. She introduces herself as a member of a forester's family, whereas the heir to the Polish throne describes himself as a squire serving the king. Their passionate love affair leads to a secret marriage which is revealed shortly before Zygmunt's coronation. However, the queen mother is furious at this turn of events as she has always planned a truly royal spouse for her son. Members of the parliament refuse to sanction the marriage, but the king's plea for recognition of his bride eventually breaks down all opposition. But the queen mother reverts to Borgia-like tactics to get rid of the unwanted daughter-in-law and the tragedy pursues.