When Mon-El and Hank bring Kara to Earth-1 to ask Team Flash for help, Music Meister puts Barry under his spell as well and traps him and Kara in a musical where they have to finish the story to escape... and if they die in the musical then they die in real life as well. This story line started at the end of Supergirl: "Star-Crossed", Season 2, Episode 16.