The CW has released an official synopsis for the two-episodes Arrowverse crossover which starts on Supergirl with 2.17 “Star-Crossed” and continues on The Flash with 3.17 “Duet”. In the States they’ll air on 20 March and 21 March, but in the UK they should air on the same night one after the other – on 28 March– unless Sky 1 does something really weird with the scheduling.
Apparently, though, like with the “Invasion!” crossover, the Supergirl episode will largely feature another independent main story, with the crossover elements only emerging towards the end.
Here’s that synopsis:
“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.”