Tonight's episode begins with a flashback to Barry and his mother watching Gene Kelly's Signin' in the Rain. Barry's abruptly woken by Cisco who has said he's been like that for weeks.
HR pages the two because a breach has opened at STAR Labs and while there, Mon-El and Martian Manhunter with an incapacitated Supergirl, who was knocked out by the Music Meister (Darren Criss) on last night's episode of Supergirl. Cisco then notices on the security camera that the Music Meister is within their facility.
Barry and Wally suit up and speed in to confront him, where Music Meister speeds over to stun Wally and incapacitate Barry, where shown Barry is in another sort of dimension where Kara Danvers/Supergirl is singing at a bar of sorts.
Kara notices Barry and the two begin talking about what's going on and realize they're without their powers. They rule out them both having a dream, and begin investigating. The two bump into Malcolm Merlyn, Cisco, and Winn, all three who have different identities in this alternate reality.
The two see the Music Meister, where he reveals that they're trapped in a motion musical. All they need to do is follow the story until it ends, then they'll be in back in their real reality. He goes on to warn them that if they die in the musical, they die in real life and the bar breaks out into singing "Put a Little Love in Your Heart."
After the musical number, they decide they'll follow along until they're confronted with a pair of gangsters and Barry is knocked out after he's hit over the head. It's revealed that Dr. Martin Stein appears to be a competing mobster of Meryln's in this reality.
Barry comes to and the two heroes have a brief heart-to-heart about their crumbling relationships. Joe West's doppelganger walks in and surveys the pair. He shows Barry a picture of his daughter and asks for their help in finding her. Cisco leads them to a location where he says Iris' doppelganger is located and the two bust the door down, only to see Mon-El and Iris kissing on the couch.
At STAR Labs on Earth-1, Caitlin tells the rest of Team Flash that it appears that the Music Meister is still the powers of Bary and Kara and the Meister can be seen using Supergirl's laser vision.
Wally, Cisco, and J'onn form a makeshift team to go out and find the Music Meister and once they locate him, he uses Barry's powers to get into a race with Wally and an epic chase and fight sequences ensues when the three each use their powers to knock the Meister out.
In the musical world, Mon-El and Iris happen to be lovers of competing mobsters. Barry tells Iris' character that her dad think she's missing and convinces her to go tell her father the truth behind her relationship.
Iris returns Joe's and Stein's characters and it's revealed the two are a couple and Iris is their daughter. She tells them about her relationship and two are understandably upset. Kara is with Mon-El, who is Merlyn's son in this reality, and he's upset as well.
Barry explains to Joe and Stein that Iris is a grown woman and should be able to love whoever she wants to love and the dads agree. Stein and Joe break out into song and once complete and Iris leaves the room, they agree to go to war with Merlyn's gang.
On Earth-1, it's shown that they've captured the Music Meister and put him in a jail cell. Mon-El and Iris confront him about releasing Barry and Kara from the reality they're stuck in. He shares that he doesn't have the power to bring them out, they have to do it themselves by finishing the musical.
In the musical, Barry and Kara begin singing a duet while gunfire erupts in the street out front, where the two gangs have started their turf way. Going out to see what's going on, Barry and Kara are both shot and their vitals on Earth-1 start going off the charts.
Cisco vibes Iris and Mon-El into the musical where they finding the duo lying in the middle of the street, where they both appear to be on the verge of death. Iris and Mon-El both kiss their respective partners and it's revealed the kiss saves them from the musical and takes them back to Earth-1.
The Music Meister escapes from his cell and reveals that he put Barry and Kara in the musical to teach them a lesson about love and he disappears.
At the Allen household, Iris and Barry begin talking about Barry's experience in the musical and Barry starts playing a song on his phone — Runnin' Home To Your — and begins singing it to Iris.
The episode ends with Barry proposing to Iris, to which she accepts and the two embrace.
Based on the characters from DC, The Flash is from by Bonanza Productions Inc. in association with Berlanti Productions and Warner Bros. Television, with executive producers Greg Berlanti (Arrow, Supergirl), Andrew Kreisberg (Arrow, The Flash), Sarah Schechter (Arrow, DC’s Legends of Tomorrow) and Aaron and Todd Helbing.
"Duet" episode of The Flash will air Tuesday, March 21 at 8 p.m. ET/PT.