Roberto and Anna's engagement is being celebrated in the house of the forest guard. However, Roberto must go to Mainz on legal matters. This is a bad omen for Anna. Soon, we see how the mermaid seduces Roberto and causes him to forget Anna. Then Anna dies of a broken heart. Anna's father, Guglielmo, mourns for his daughter. When Roberto repentantly returns, Anna's spirit appears and both are surrounded by other spirits. They compel Roberto to dance with Anna until he dies.