and Mon-El of Earth-38 arrive on Earth-1 with a comatose Kara Danvers, in hope that Barry Allen and his team can revive her. They warn Barry that the escaped prisoner, Music Meister, claimed he would be coming for him too. Meister attacks and places Barry in a similar coma. Barry wakes up to find himself without his powers in a musical dreamworld where he discovers Kara singing in a nightclub and where everyone resembles people they know in the real world. Meister tells Kara and Barry that if they follow the script, they will return to the real world. He warns them, however, that if they die in this world, they die in real life. Barry and Kara are forced to work as singers in a nightclub run by gangster Cutter Moran. Cutter's rivals Digsy Foss and his husband have their men kidnap Barry and Kara and tell them to find their daughter, Millie Foss, whom they believe is being held hostage by Cutter. Barry and Kara find Millie in a forbidden relationship with Cutter's son, Tommy Moran. They convince the pair to reveal their love, which helps Barry and Kara to realize their own mistakes. Digsy, his husband and Cutter seemingly give their approval, but subsequently decide to go to war. Meanwhile in the real world, Meister is revealed to have been stealing Barry and Kara's powers, using them to rob a bank. Cisco Ramon, Wally West and J'onn arrive to fight Meister. They defeat him and lock him up in a S.T.A.R. Labs cell. Back in the musical reality, Barry and Kara prepare to sing an original song to appease Cutter, but the gang war begins outside. Barry and Kara are shot in the crossfire and are dying, but Cisco, Mon-El and Iris vibe into their world to save them, allowing Barry and Kara to admit their love for Iris and Mon-El respectively. They wake up in S.T.A.R. Labs. Meister escapes his cell, revealing to Barry and Kara that he just wanted them to realize their love before leaving. Kara, J'onn and Mon-El return to Earth-38 and Barry and Iris move back in together. Barry serenades and re-proposes to Iris, who accepts.
Filming of the episode occurred from February 1 until February 15, 2017, and was directed by series veteran Dermott Daniel Downs.
Music
In January 2017, Rachel Bloom revealed that she had penned an original song for the episode, which was titled "Superfriends", and was set to be sung by Gustin and Benoist. Bloom explained that, "as soon as I heard that they were doing a musical crossover, I e-mailed Mark Pedowitz who connected me to Greg and Andrew . I immediately offered them my services. As soon as they picked one of my song ideas, I hopped on the phone with my old Robot Chicken boss Tom Root and... based on that brainstorm, I wrote up the song 'Superfriends'. I am so excited to contribute more to the upward trend that is musicals in television and film. Music can be one of the most amazing and efficient forms of storytelling and character development. Also, it was really fun to write a comedy song for two superheroes." The song was later retitled "Super Friend". Additionally, Benj Pasek and Justin Paul, winners of the Academy Award for Best Original Song for writing the lyrics for "City of Stars" in La La Land, were the authors of another song written for the episode, "Runnin' Home to You", which was performed by Gustin. Kreisberg commented, "Benj and Justin are the premiere songwriting duo of our time... To have them be a part of our musical episode is beyond our wildest expectations. We hope everyone falls in love with the song they wrote for us as much as we did." A soundtrack, titled, The Flash – Music from the Special Episode: Duet, was released on March 22, 2017 from WaterTower Music containing the songs featured the episode.
Release
Broadcast
After beginning in the ending scene of the March 20, 2017 episode of Supergirl, "Duet" aired in the United States on The CW on March 21, 2017. It was simulcast with the US broadcast in Canada on CTV Two.
Marketing
In early March 2017, a promotional poster for the episode was released, with it being marketed as "Dynamic Duet", a play on words of the phrase "dynamic duo". In Australia, where The Flash and Supergirl both air on Fox8, the crossover was marketed as a two-night event called "DC Musical Crossover".
Critical response
Jesse Schedeen of IGN gave the episode an "amazing" 9.6 out of 10. He felt the premise was "goofy" but did a good job incorporating the existing continuity from both series, such as building "directly on recent romantic tribulations in Barry and Kara's lives." He enjoyed the team-up with Kid Flash, Martian Manhunter and Vibe, and was glad the writers did not "attempt to force in a romantic subplot involving Barry and Kara. As their "Super Friends" number showed, they're better as friends than lovers." The A.V. Clubs Scott Von Doviak awarded the episode a "A−", saying there was good chemistry between Gustin and Benoist and that Gustin was able to "reconnect with the character's endearing side." Von Doviak concluded, "It's probably logistically untenable for them to ever be more than 'super friends,' but if this show ever needed the positive energy they generate together, now was certainly the time."
