The Flash Supergirl Musical Crossover: 14 Easter Eggs & References In 'Duet' You Need To See

4. Crazy Ex-Superfriend

One of the two big original numbers written just for this episode, Super Friend is the instalment's biggest homage to the golden age of musicals, full of glitz, glamour, and toe-tapping, with Barry even wielding a cane at one point.

The song was written by Rachel Bloom, who comes from, if not exactly the Arrowverse, then at least the CW-verse, as the star and creative genius behind the brilliant Crazy Ex-Girlfriend. Involving talent like Bloom is one of the ways the episode really stands it, giving the songs true quality that might otherwise be lacking had the regular writers written them (and that's not a criticism).

As Kara sings, the title of the song has a double meaning. Outside of the show, however, it's more like a triple meaning, referring not only to their super friendship and super powers, but also the animated TV series Super Friends, which ran from 1973-86, and regularly featured The Flash (while Supergirl later appeared in comics and merchandising). It's also the name Winn considers calling the group in the Supergirl pilot.

The other original number comes right at the end, with Barry singing Runnin' Home To You. Given the rejuvenation of the musical genre thanks to the success of La La Land, it's fitting that this song comes from the songwriting duo who provided the lyrics for that movie, Benj Pasek and Justin Paul.