“Duet” is episode seventeen of season three of The Flash.
This episode opens 18 years ago and young Barry is watching Singing’ in the Rain with his mother. Forward to the present, and he’s watching it again, this time at Cisco’s. In the previous episode, “Into the Speed Force”, Barry suggested to Iris that they should spend some time apart. Cisco wants Barry to get off the couch and do something, as he hasn’t done much since whichever it was of them broke off the engagement. Then they both get a message from H.R. and head to the breach room. There, Mon-El carries an unconscious Kara through the breach, accompanied by J’onn.
In the Supergirl episode, “Star-Crossed”, Winn told Kara he had got the interdimensional extrapolator that Cisco gave her in the DC’s Legends of Tomorrow episode “Invasion!” working. Kara – who had split up with Mon-El – said she could do with a post-breakup inter-dimensional vacay. Then a prisoner escaped his bonds, did something to Kara, stole the, extrapolator, activated it and went through the breach, leaving the device behind, saying he had to chase down the fastest man alive. Kara, meanwhile, found herself about to go on stage. And sing.
J’onn tells them that they don’t know what happened to Kara, but whoever did it came to this world. Caitlin hooks Kara up and says that her vitals are normal. Mon-El states they need to find whoever did this to Kara and punch them repeatedly. He gets to punch them first. Barry asks Mon-El who he is, and he replies that he’s Kara’s friend. More than friend. They’ve been kissing a lot. It’s a recent thing. J’onn thought the two of them broke up and Mon-El shushes him. H.R. puts his foot in it by saying he knows better than to mention break-ups in front of Barry and Iris. Thereby mentioning break-ups in front of Barry and Iris. J’onn explains that an alien prisoner escaped, did something to Kara, then came through to this world. Given that he stated he was going to find the fastest man alive, they think he was coming for Barry.
J’onn also says that the man seemed to just materialise out of thin air. Which, according to Cisco, someone has just done on their security cameras. Barry speeds off and H.R. encourages Wally to follow. The man greets Barry – by name – and then Wally too. What he wants with Kara is the same thing he wants with all of them. To teach them a lesson. He then speeds across the room, knocking Wally down, grabs Barry by the throat and does the same thing with his eyes that he did to Kara.
Barry comes to in the same nightclub as Kara. It’s easy to see it’s the same club, as she is up on stage, singing Moon River. Kara notices Barry and, afterwards asks him if it’s really him. Neither know what is going on. Barry suggests they are dreaming, but they are both having the same dream. Barry explains that Kara’s boyfriend brought her through to his world. She replies that Mon-El isn’t her boyfriend. He’s a boy who isn’t her friend. When Kara suggests clicking her heels together three times, Barry thinks she’s being serious and that it might work. She wasn’t.
Next they are spoken to by Malcolm Merlyn. Except he isn’t. He’s Cutter Moran, the owner of the club, and they work for him. He’s also quick with a knife. Cutter wants them to perform something original. Neither Kara nor Barry has any powers. Then they are introduced to Grady, the pianist. Who looks just like Winn. And Pablo. Who looks like Cisco. They appear to be in a musical. Both Barry and Kara like musicals. The man who brought them here, the Music Meister, states that he actually didn’t. They created this world, it’s in their heads. And they’re lucky that they don’t like war movies or space operas.
The Music Meister isn’t really here. He’s in the real world, and Central City is his for the taking. They are welcome to stop him. If they can get out. It’s a musical so all they have to do is follow… no, not the yellow brick road as Kara suggests. The script. Oh, and if they die in this world, they die in the real world. Kara states she has reached her limit if magical creeps, referring to the events of “Mr. & Mrs. Mxyzptlk”. Then the Music Meister leads everyone, bar Kara and Barry, in a musical number before vanishing.
Kara and Barry fail to find him, and ask what they should do now. They are singers and a gangster wants them to perform. This sounds like s simple plot. Then men with guns appear. Kara misses being bulletproof. The men – who are led by not-Professor Stein – cart the two of them away. Where they meet Digsy Foss – who is not Joe west. He wants them to find his daughter, not-Iris but Millie, who was last seen at Cutter’s club.
Pablo shows the two to where Millie is. Kara is glad that they haven’t seen anyone like Mon-El. Thereby guaranteeing they will. Not-Mon-El is Tommy, Moran, Cutter’s son. Who is in love with Millie. Their fathers don’t like each other. So, just after an episode called “Star-Crossed”, they have an episode about star-crossed lovers. Barry mentions West Side Story, Kara The Fantasticks.
In the real world, both are being drained, according to Caitlin, of the speed force for Barry and solar radiation for Kara. Caitlin thinks the guy who took them is stealing their powers. Because he is. Wally and Cisco go after him and J’onn volunteers as well. Cisco says that, although he digs J’onn’s secret agent thing, this is a special situation and they need someone with… green skin and a cape as J’onn changes form.
Barry and Kara talk about their respective relationships, and see some similes between the ones in their imaginary world and their real relationships. Music Meister is definitely after something, but it’s not what might be expected.
So… this is a different episode. There’s quite a bit of singing in it for one thing. Fortunately, a lot of the cast has experience with singing in musicals and elsewhere. Grant Gustin (Barry Allen), Melissa Benoist (Kara Danvers) and Darren Criss (Music Meister) all appeared in Glee together. This can be considered to be a break from the usual grimness of the Savitar plotline, and actors from the four Arrowverse series appear – even if not in their usual roles.