Fantasy television has always been able to get away with things that shows based in reality can't necessarily pull off, such as musical episodes. While sometimes there have been shows where every episode is a musical in spite of being reality-based - such as Glee and Smash - single musical episodes certainly lend themselves to shows where the audience already knows to expect the unexpected. One of the most famous examples is the Buffy the Vampire Slayer episode 'Once More With Feeling'. And now the long awaited musical crossover between Supergirl and The Flash is close at hand. Superhero shows certainly fall into the fantasy genre that lends itself well to the musical episode. After all, when the heroes are aliens or have super speed, it's not so crazy to accept them breaking into song.

And these shows' writers know it. Over the last few years, they have given several members of their casts the chance to sing on screen. On The Flash, Barry sang karaoke in one episode, and on Earth-2, Joe West was apparently a lounge singer. In the past year, on Legends of Tomorrow Martin Stein has gotten to sing a couple of times as well, both when he was undercover at different times in history. However, in the upcoming episode 'Duet', it will not be only one person singing in one scene. This will be a complete musical episode, with Victor Garber and John Barrowman appearing alongside the recurring cast members of The Flash and Supergirl.

In a bit of a tease for the episode, Grant Gustin revealed a fun new poster on his Instagram page (see below), which indicates that things will not be all sunshine and happiness for Barry Allen in 'Duet'.

Based on what we know about 'Duet' already, it seems the hand controlling the marionette would belong to The Music Meister. The villain of the week, played by Darren Criss, will be trapping both Barry and Kara in a musical movie, with them needing to survive until the end in order to escape. Not only is he controlling the heroes by trapping them in the fantasy, but also by making them participate. As the Meister says in the 'Duet' trailer, Barry and Kara have to "follow the script."

While trapped in The Music Meister's fantasy, Kara and Barry will also be seeing some familiar faces. Joe West and Martin Stein will appear to be there, but as rival gangsters. Iris will appear as a gangster's daughter, and Winn as a pianist. Malcolm Merlyn and Cisco are also part of the fantasy, and possibly other friends and foes are, too.

Since Barry and Kara both have huge dangers to be dealing with - Cadmus for her and Savitar for him - odds are that they will not appreciate having to deal with The Music Meister's fantasy world. Neither really has time for the distraction. Still, both shows have a number of talented singers in their casts, and the audience may enjoy an hour of levity, before both shows return to the high stakes and danger leading up to their season finales.

Supergirl continues with ‘Star-Crossed’ on Monday, March 20 at 8 p.m. EST, followed by The Flash on Tuesday, March 21 in the same time slot with ‘Duet’.

Source: Grant Gustin