Barry and Kara are both superheroes, but just because they have superpowers doesn't mean they're particularly super when it comes to love. On this week's special The Flash — Supergirl crossover episode, "Duet," both Barry and Kara were reeling from fresh breakups. And, as if that weren't enough to deal with, they both got sucked into the Music Meister's musical fantasy world, without any way out. While Kara and Barry were both rendered incapacitated by the Music Meister, it turns out he wasn't really much of a villain after all. When all was said and done, the Music Meister taught Kara and Barry about love on The Flash.
Music Meister put Kara and Barry's lives at risk by bringing them into an alternate reality where they were stars of a movie musical involving a gang war. But, he did it to teach them about true love, so, it's cool, right? Even though the Music Meister's spell also gave him Flash and Supergirl abilities, he didn't really want them. In fact, he gave them back willingly once Barry and Kara learned their lesson: they needed to believe in love. The Music Meister manipulated Barry and Kara into understanding why they made mistakes in their respective relationships: Barry by letting Iris go, and Kara by not trying to see things from Mon-El's perspective.
In the end, it was true love's kiss that broke the spell and brought Kara and Barry safely back to Earth-1, with Mon-El and Iris entering into the musical world to save them. All Music Meister wanted to do was show Kara and Barry that they should hold on to the love they have, and Mon-El and Kara returned back to their Earth happily together. Hopefully, the Music Meister will return and treat us to another delightful musical episode soon.