Barry and 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. The crossover starts on Supergirl S02E16 Star-Crossed (I).
Singin' in the Rain — Gene Kelly
In the flashback, Barry and Nora watch Singin' in the Rain together; she tells him musicals always make things better. Back in the present, Cisco finds Barry watching the movie again.
Moon River — Melissa Benoist
Barry arrives at a club in the Music World to find Kara singing at the club. Kara's eyes light up as she realizes that he is there but she's unable to pull herself away from singing.
Meet the Music Meister — Blake Neely
The Music Meister introduces himself to Barry and the gang; (repeats) the Music Meister tells Kara and Barry how to can get out of the music world; (repeats) the Music Meister says goodbye to the Flash/Supergirl teams.
Put a Little Love in Your Heart — Darren Criss & jeremy jordan & Carlos Valdes & John Barrowman
Music Meister and everyone else in the Music World sings to get Kara and Barry to go along with the script then disappears.
More I Cannot Wish You — Jesse Martin & Victor Garber & John Barrowman
Millie's fathers sing to her about true love; Cutter sings to Tommy about love.
Super Friend — Melissa Benoist & Grant Gustin
Barry and Kara are The Flash and Supergirl and they are the 'Super Friends;' Kara and Barry sing an original song together during practice.
Runnin' Home to You — Grant Gustin
Barry sings to Iris as he proposes to her in a romantic way