The CW just released the official summaries for the crossover episodes, which air on Monday, March 20th, and Tuesday, March 21st. Here they are:
Meanwhile, Winn’s (Jeremy Jordan) girlfriend, Lyra (Tamzin Merchant), gets Winn in trouble with the law. Maggie (Floriana Lima) attempts to help Winn but old loyalties get in the way. The Music Meister (Darren Criss) attacks Supergirl.
In The Flash‘s “Duet”: “Flash/Barry (Grant Gustin) and team are surprised when Mon-El (Chris Wood) and Hank Henshaw (David Harewood) 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.”
Grant will sing their new original song, “Runnin” Home to You.