- For the titular character, see Mxyzptlk.
- "Kara, sweetest, like I said, I'm your one true love, your soulmate, your one true pairing, as the kids say. My name is Mxyzptlk and I love you, Kara Zor-El.""
"Tell me, will you marry me?"
- —Mxyzptlk proposes to Kara Danvers
When Mr. Mxyzptlk, a magical imp, shows up on Earth and declares his love for Kara, she tries to let him down easy, thinking him harmless. However, he decides it's a challenge and starts to wreak havoc on National City. Mon-El has seen beings like Mr. Mxyzptlk on Daxam and knows how dangerous they can be. He and Kara argue over how to get rid of Mr. Mxyzptlk, further straining their relationship. Meanwhile, Alex and Maggie celebrate their first Valentine's Day together.
As Kara and Mon-El are baffled about Mr. Mxyzptlk, he immediately starts swooning over Kara and when Mon-El attempts to make him stop, he sends him to DEO, where he says "I'll kill him." Mxy proposes to Kara and even puts her in a wedding dress. Kara is finally able to refuse his proposal, but Mxy only believes she is playing hard to get, and then leaves, returning everything normal.
At the DEO, Mon-El reveals that similar creatures have been on Daxam before, and whenever they showed up, they killed them. Kara refuses to do that and J'onn has Winn look about him and also see if DEO has anything that could help against Mxy. Kara also assures Mon-El that she can handle him.
Alex and Maggie are spending time together and Maggie reveals how she hates Valentine's Day. Alex says the same, but in truth she likes it, just as Kara does.
The next day, an armed robbery takes place and Kara swoops in to stop it as Supergirl. Suddenly Mxy appears and telekinetically turns the robbers guns against them, forcing Kara to jump in front of them so that Mxy won't kill them. Mxy claims that he can give Kara anything she wants and assist her with anything, but if she doesn't marry her, he will unleash more chaos.
Later, Alex asks Kara for tips about how to spend Valentine's Day with Maggie, as she hates it. Winn had discovered some things that could help against Mxy, including an amulet that supposedly severs Mxy's connection with the Fifth Dimension. Mon-El becomes intrigued by it.
The following evening Alex tries to surprise Maggie, but she walks away disgusted. Kara and Mon-El are spending their evening in her apartment, when they suddenly hear ruckus outside. They look from the window and see the Parasite wreaking havoc. Both race down to deal with it, but they get knocked out. Suddenly, above everyone, a man is seen flying, which seems to be Superman. He takes out the Parasite and lands, but it's actually Mxy wearing a similar suit as Superman. Kara and Mon-El realize that he conjured the Parasite in order to show off for Kara. Mon-El loses his temper and tries to get Mxy to spell his name backwards, which baffles Kara. Mxy again sends Mon-El away and gives Kara a final warning: marry him or face the consequences.
At the DEO, Kara scolds Mon-El for not telling her that getting Mxy to say his name backwards is a way to defeat him. Mon-El reasons that it never works, since Mxy isn't stupid. Kara then tells Mon-El to leave this as she handles this alone. Mon-El then takes the amulet and races to face Mxy himself. As Kara is starting to conclude that maybe Mon-El is right. J'onn discovers that the amulet is missing. Kara instantly realizes Mon-El took it.
Mon-El finds Mxy and challenges him. Mxy teleports them into a theater wearing suits from 18th century and muskets. Mon-El shoots him, but the gun is not really loaded. Mon-El uses the amulet, which prevents Mxy to use his abilities, but in the end it's no use, as Mxy is able to subdue him. Kara arrives just as Mxy is about shoot Mon-El with a lead bullet. Kara seemingly gives up, and agrees to marry Mxy, just as long he lets Mon-El go. Mxy immediately prepares for the ceremony, but Kara gives him a condition that they will marry tomorrow at the Fortress of Solitude, where she can wear her mother's dress. Mxy agrees.
The next day Mon-El arrives at Kara's apartment and tries to convince Kara not to marry Mxy, but Kara is adamant and claims it wouldn't work with them, and that Mxy's powers could be helpful for her as Supergirl and Mon-El leaves.
At the Fortress of Solitude, Mxy arrives but Kara isn't there yet. She finally arrives in her Supergirl suit with a glass of orange juice. As Mxy wants to get on with the wedding, Kara rejects him, and says she never intended to marry him; rather she played him in order to trap him inside the fortress, so he wouldn't hurt anyone else. Mxy gets angry and animates the statue of Jor-El to attack Supergirl. She destroys it while apologizing to her uncle. Then, Kara sets the Fortress' reactor core to self-destruct. Mxy believes she is bluffing, but Kara is dead serious that she would rather die than marry him. Mxy demands the override code but Kara doesn't give it. As time is running out, Mxy starts panicking as he can't figure out the override code himself. Finally, Kara shows it to him and Mxy types it himself. The self-destruct sequence stops and Mxy is relieved, but Kara then shows that the override code was Mxy's full name spelled backwards. Outraged that Kara managed to trick him, he tries to attack her, but the effect of the spelling is starting to take effect. As Mxy disappears, he says that all he ever wanted was to have someone to love him, but Kara says that love can't be forced on anybody; it needs to come on its own.
Alex arrives at her apartment, where she finds a note and a gift from Maggie. Putting on the dress and going to the place Maggie had told, Alex then gets her own prom.
At Kara's apartment, Mon-El arrives and is amazed that Kara managed to trick the most notorious trickster in the universe. As they talk about what happened, Mon-El, thinking that Kara was serious when she said those things to him, is going to leave Kara for good, but Kara stops him and admits she didn't mean it; she just said those things to protect him, and Mxy could have been listening. Kara and Mon-El are then finally able to do what Mxy interrupted and kiss, turning it into a passionate moment between the Kryptonian and the Daxamite.
- This episode is a loose adaptation of Supergirl and Mr. Mxyzptlk's first meeting in Action Comics #291: The Bride of Mr. Mxyzptlk! (August, 1962). In that story, Mr. Mxyzptlk tried to coerce Supergirl into marrying him using his magic.
- Mxyzptlk tries to serenade Kara by briefly singing "A Whole New World" from Disney's Aladdin.
- Mxyzptlk's attempt at shooting the armed robbers with their own guns is a direct reference to a scene in the first entry in the X-Men film series, further driven home by Mxy saying he "saw this in a movie once".
- Mxyzptlk's idea of a duel in the theatre is a direct reference to the duel between famous Americans Alexander Hamilition and Aaron Burr, furthermore the musical Hamilton, in which Hamilton and Burr duel with pistols, the latter taking the former's life (a fact referenced by Mxy referring to Mon-El as Hamilton).
- Additionally, Mxyzptlk suggests that Mon-El not "throw away [his] shot", referencing the third song of Hamilton, "My Shot".
- After Mxyzptlk grabs the pendant from Mon-El, he exclaims "Strike that. Reverse it.", quoting Willy Wonka from the 1971 film Willy Wonka & the Chocolate Factory.
- Supergirl's line "You wanna get nuts? Let's get nuts!" is taken from the 1989 Batman film.
- Mxyzptlk calling Kara a "nasty woman" is possibly a reference to the Presidential debate between Hillary Clinton and Donald Trump that took place on October 19, 2016, during which Trump called Clinton a "nasty woman".
- When Mr. Mxyzptlk is dressed as Superman, he lands in front of Smallville’s Daily Planet building.
- Mxyzptlk calling Mon-El "McGurk-El" seems to be a callback to Superman: The Animated Series where in his debut episode, Mxy was shouting the name "McGurk" during several of his entrances.
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