Renée Fleming sings one of her signature roles, the title character in Dvořák’s sumptuously melodic Rusalka. The story of the opera, which is about a water spirit’s tragic romance with a human prince, is drawn from several folktale sources including Hans Christian Andersen’s “The Little Mermaid.” Star conductor Yannick Nézet-Séguin leads a cast that also includes Piotr Beczala as the handsome Prince whom Rusalka yearns to love; Dolora Zajick as the cackling swamp witch Ježibaba; Emily Magee as the Foreign Princess, Rusalka’s rival; and John Relyea as Rusalka’s father, the Water Sprite..