Beware: Spoilers may follow.
In all four Arrowverse shows there is at least one actor in each of them who has some sort of musical background. Some of them have been given a chance to showcase their talent in other episodes. However never has there been a full-on musical episode. This Supergirl-Flash crossover was an attempt to do that bringing the best singers from each show together into an upbeat and all-around entertaining episode.
The story is that Barry (Grant Gustin) Allen and Kara Danvers (Melissa Benoist) have broken off their relationships with Iris West (Candice Patton) and Mon-El of Daxam (Chris Wood) respectively. When Kara while on her Earth falls into coma by a villain named Music Meister (Darren Criss) Mon-El and J’onn J’onzz (David Harewood) bring her to Star Labs for help. However the Music Meister puts Barry in the same coma. Kara and Barry wake up in an alternate reality without their powers and their life is like a musical. A lot of the people in this reality that the two know in the real world are entirely different people as well. The Music Meister claims that if they want to get out they must make it all the way through the musical and that if at any point they die in this world they die in the real world as well.
While there is a fun subplot with Wally West (Keiynan Lonsdale) a.k.a Kid Flash, J’onn, and, Cisco (Carlos Valdes) taking on the Music Meister in the real world the main focus of the story is in the alternate reality. The story does a good job in paying homage but also satirizing 40s musical. With our two mains inside of it, there is somewhat of a self-awareness factor to it all making it all the more engaging. And being in the alternate world does lead to Kara and Barry making important realizations. The scene where Kara realizes why Mon-El kept his royal lineage a secret from her after calling out Cutter for not understanding why his son didn’t tell his dad the truth is conveyed effectively through just her expression.
Much like Welcome to Earth 2 a lot of the humor is derived from Kara and Barry’s reactions to meeting the alternate versions of several people they know. The difference here from that episode is that, in this episode the alternate versions of the characters have the same faces but entirely different names and (sometimes) personalities. The reveal that alt. Joe West named Digsy Foss (Jesse Martin) and alt. Stein (Victor Garber) are romantic partners was hilarious as was alt. Malcolm Merlyn named Cutter Moran (John Barrowman) as a nightclub owner with a liking towards stabbing people who annoy him. All the actors looked like they were having fun in playing these alternate versions. Candice Patton in particular seemed to be having a blast as Millie. Having alt. Mon-El be named Tommy and making him Moran’s son was a nice touch for those who watch Arrow.
Darren Criss as the villain gave a fun performance (although I do wish they could have brought back Neil Patrick Harris but he was busy with A series of Unfortunate Events). He gives the character a theatrical yet whimsical edge that while it doesn’t fit with the mythos of the Music Meister, matches up well with the tone of the episode.
But of course no musical is complete without…well music. Director Dermott Downs knew how to bring a great deal enjoyment out of each song. Benoist starts things off great with her beautifully sung rendition of Moon River. Put a Little Love in your Heart is amazingly sung and choreographed by Criss in addition to Barrowman, Valdes, and Jeremy Jordan (who portrays Winn in Supergirl). More I Cannot Wish You is sung amazingly well all around. Super friend is a fun and silly song, which builds on the friendship Kara, and Barry have built up. The final song Running Home to You is also sung romantically well by Gustin (and in fact was written by the song writers of La La Land) and is easily one of the best romantic moments between Barry and Iris.
If there is one real complaint it was that there weren’t enough music numbers. Criss barely gets a number outside of Put a Little Love in Your Heart which is a shame because he is the one who ignited this whole thing and is a major star of Glee (since with Melissa and Grant who were also on Glee it is sort of a reunion). But there are only so many songs that could be fit into 43-minute episode (although it would have been nice to see it go on longer). Also Tom Cavanagh who is also a good singer didn’t get an alternate counterpart. If he had he would have made a fun addition to various Harrison Wells out there.
Duet proves that it can take such a silly concept and make it entertaining through its characters. It also ties up two romantic conflicts that easily could have taken weeks to resolve. And it takes the best singing talents from each of the four Arrowverse shows and combines them into something that the cast was definitely having fun putting together.