Though the story starts in the Supergirl episode “Star-Crossed” on March 20, most of the action takes place March 21 on The Flash, in an episode that itself is called “Duet.” The episode reunited former Glee co-stars Melissa Benoist, Darren Criss, and Grant Gustin while they’re at it. Here’s how the network describes it:
DARREN CRISS (“GLEE”) GUEST STARS AS THE MUSIC MEISTER IN THE FLASH AND SUPERGIRL MUSICAL CROSSOVER – Barry (Grant Gustin) and team are surprised when Mon-El (guest star Chris Wood) and Hank Henshaw (guest star David Harewood) arrive on their Earth carrying a comatose Supergirl (guest star Melissa Benoist) who was whammied by the Music Meister (guest star Darren Criss). 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. Dermott Downs directed the episode with story by Greg Berlanti & Andrew Kreisberg and teleplay by Aaron Helbing & Todd Helbing (#317). Original airdate 3/21/2017.
The network has just released some photos which you can find below. Enjoy!