- Time: The ninth or tenth century
- Place: The Slavic lands of the Baltic sea-coast, in the city of Rethra, near the Labe (Elbe) River, in modern-day Germany.
Voyslava has killed Mlada, Yaromir's bride, to have him for herself. With the help of Morena, the goddess of the underworld, she has captivated Yaromir. But he sees the murder in his dreams.
At the midsummer festival, the people dance, while the spirit of Mlada intervenes between Yaromir and Voyslava.
By night Mlada leads Yaromir up Mount Triglav, where the dead gather, before the Witches' Sabbath in which Yaromir is shown a vision of Cleopatra.
Yaromir, at the Temple of Radegast, is shown by the spirits that Voyslava is guilty. She confesses her sin and he kills her. Morena, with whom Voyslava had made a compact, destroys the temple and the city of Rethra, but Yaromir is united with Mlada in heaven.