Both "Supergirl" and "The Flash" have traveled deep into space and traversed the multiverse, but they're about to encounter a world unlike any they've seen before. In "Duet," Barry and Kara will find themselves in a musical dream world created by Darren Criss' Music Meister. Music Meister's vision for Supergirl and Flash involves several of their supporting characters, who will take on all-new roles for the duration of the episode. For Iris and Mon-El specifically, that means a new romance as well.
“Barry and Kara envision this world where Iris’ character and Mon-El’s character are very, very deeply in love with each other. And that obviously throws [Barry and Kara] off, because those are their love interests,” revealed "The Flash's" Candice Patton.
In the musical crossover, Patton will play Millie, who she described as the “fast-talking, brash daughter of a mobster.” Chris Wood, on the other hand, will play Tommy, the son of John Barrowman's as-yet-unnamed mob boss character. The circumstances in Music Meister's dream world will find the two “fighting for their love, because their families don’t want them to be together.”
“Music Meister is almost a sadistic, wannabe Cupid,” added "Supergirl" star Jeremy Jordan. “His way of telling a love story is by putting you in a trance and making you fight your way out.”
“The Flash” and “Supergirl” musical crossover will start with a lead-in scene on the March 20 episode of “Supergirl,” and continue in full on the March 21 episode of “The Flash.” The episode will include musical numbers from “The Flash” stars Grant Gustin, Jesse L. Martin and Carlos Valdes; “Supergirl” stars Melissa Benoist and Jeremy Jordan; and “Legends of Tomorrow” co-star Victor Garber and “Arrow” regular John Barrowman; along with “Glee” alum Darren Criss as the Music Meister, the episode’s antagonist.