The operetta takes place in Vadstena during Erik XIV's reign, and is about how local fishermen try to exploit the insane SwedishDuke Magnus, who lives in Vadstena castle, by making him fall in love with a fisherman's daughter who he believes is a mermaid. The model of the plot is a historically unsettled event, when the real Magnus was said to have seen a mermaid outside his window and fallen into the moat. The event led to Magnus being introduced as the protagonist of medieval ballads and songs about alluring sea creatures. [2]