Duet

"Everything is better in song. When you speak, it's just words. But when you sing, you open up your soul and let who you really are shine through."

- Nora Allen

"Duet" is the seventeenth episode of the third season of The Flash, and the sixty-third episode overall. It aired on March 21, 2017 and is the main part of a musical crossover between The Flash and Supergirl, which was set up at the end of the sixteenth episode of the second season of Supergirl.

Synopsis
Barry and the team are surprised when Mon-El and Hank Henshaw arrive on their Earth carrying a comatose Supergirl who was whammied by the Music Meister. Unable to wake her up, they turn to Team Flash to save her. However, the Music Meister surprises The Flash and puts him in a similar coma, one that Team Flash can’t cure. Kara and Barry wake up without their powers in an alternate reality where life is like a musical and the only way to escape is by following the script, complete with singing and dancing, to the end.

Trivia

 * Three pre-existing songs were sung:, and ; while two original songs were written especially for the episode:  and.
 * All 5 songs were released on a digital soundtrack, along with the instrumental,, as well as a special guitar version of "Runnin' Home to You"
 * Tommy Moran's name is likely a reference to Tommy Merlyn, the late son of his father's counterpart, Malcolm Merlyn.
 * A young Barry and Nora watch the 1952 musical film Singin' in the Rain, watching the scene featuring the title song.
 * Numerous references are made to the 1939 film The Wizard of Oz.
 * When thinking of ways to get them out of their dream, Kara suggests that she "click [her] heels together three times", referencing the way in which protagonist Dorothy Gale clicks her slippers and is magically transported home from Oz.
 * After they begin meeting people who resemble those they know, Kara likens it to The Wizard of Oz, quoting the film, "and you were there, and you were there". In the final scene of the film, after Dorothy returns home, she sees people who resemble people she'd met in Oz.
 * Music Meister mentions the countless times that Kara watched The Wizard of Oz with her adoptive parents.
 * As the Music Meister goes to tell them to follow the script to survive, Kara thinks that he's going to suggest to "follow the yellow brick road", advice given to Dorothy Gale by Glinda in the film.
 * After Millie refuses to go home, Kara claims that there is "no place like home", referencing the line spoken by Dorothy in order to travel home.
 * Barry describes the dream as "curiouser and curiouser", a phrase used in the 1865 novel Alice's Adventures in Wonderland by Lewis Carroll.
 * Music Meister mentions Barry having watched Fred Astaire and Frank Sinatra with his mother, two big names in musical history.
 * Discovering Tommy and Millie's star-crossed romance, Barry likens the story to West Side Story, though Kara sees it more as The Fantasticks.
 * Tommy and Millie's names are also likely a reference to Tony and Maria from West Side Story.
 * Grant Gustin, Melissa Benoist and Darren Criss all previously appeared on the series Glee.
 * Darren Criss alluded to this in an interview but stated that the crossover/reunion was also connected to some of the cast and crew being alums of the University of Michigan - himself, Carlos Valdes, and song writers Benj Pasek and Justin Paul, to name a few - and the fact that many of the actors either majored in musical theatre or have very close ties to musicals and the music industry.