Barry's on Cisco's couch drowning his sorrows in Singin' in the Rain. He's loved musicals since he was a kid and used to watch them with his mom. Cisco tries to cheer him up, but it's not working. H.R. calls and they rush to STAR Labs. A breach has been opened. Mon-El and J'onn come through with an unconscious Supergirl. They explain that she was whammied by an escaped alien. They followed him to Earth-1.
The Music Meister arrives looking for Barry. He puts him under the same spell as Kara. Then, he takes their powers, and begins wrecking havoc on Central City. Cisco, J'onn, an Wally team up to stop him while Caitlin tends to Barry and Kara. Music Meister knows Wally's been out of sorts, and he's been afraid since he was trapped in the Speed Force. Wally insists he's fine. The three manage to take down Music Meister, and transport him to a cell. Iris and Mon-El try to negotiate with him to bring back Barry and Kara. He tells them only they can do that.
Barry finds himself at an old fashioned jazz club. Kara is on stage singing. The two compare notes about their run-ins with Music Meister. They also talk about their failed love lives. Kara's mad at Mon-El for lying to her, and Barry feels guilty for pushing Iris away. But he also believes it's the best solution for the time being. Music Meister shows up and informs them that they need to follow the script. Once they complete the movie musical, they can go home.
Several familiar faces pop up throughout. Joe and Professor Stein are married mobsters. Malcolm Meryln is their rival. Cisco and Winn both work at the club, as a waiter, and keyboard player, respectively. Stein and Joe task Barry and Kara with finding their daughter. They find Iris and Mon-El holed up together. They're in love, and they don't want their fathers to know. Barry and Kara attempt to solve their problems. In doing so, they realize the advice they're giving the fake versions of Iris and Mon-El applies to the real life problems.
But the mobsters don't care for love. They stage a shootout, and Barry and Kara are caught in the crossfire. Both are near death when Cisco vibes the real Iris and Mon-El into the world. Iris and Barry profess their love and share a kiss. Kara forgives Mon-El and he promises not to lie to her again before they kiss. They're transported back to the real world. Music Meister admits he did all of this to save their love lives. He disappears. Kara and her friends return to National City, and Barry proposes to Iris again. She accepts.