Tristan und Isolde opens the Met season in a new production by Mariusz Treliński (the director responsible for the 2014-15 season’s double bill of Iolanta and Bluebeard’s Castle), and will be well served by a cast of outstanding Wagnerians: Nina Stemme as Isolde, Stuart Skelton as Tristan, Ekaterina Gubanova as Brangäne, and René Pape as King Marke, with Sir Simon Rattle conducting, in one of his rare appearances at the Met.