The episode opens with Mxyzptlk proposing to Kara after making candles and a string quartet appear out of thin air. Mon-El tries to step in, saying that Kara's with him, but Mxyzptlk simply teleports him to the DEO...in nothing but his underwear. Kara tries to slow Mxyzptlk down after he puts her in a wedding dress, but Mxy assumes that she's playing hard to get. He says that once he falls for the all-powerful Mxy, she can never go backsy.
J'onn explains that Mxy is a 5th dimensional being, and that both Mars and Daxam have come into contact with them before. Mon-El warns that 5th dimensional beings party hard, but are quite dangerous. Afterwards, J'onn says that he needs to send M'gann a Valentine's Day message and Kara promises Mon-El they can get back to their previous "conversation" after Mxy goes away.
The next morning, Alex finds out that Maggie hates Valentine's Day, which is problematic because Alex clearly loves the holiday.
Winn tries to get drunk at the (STILL UNNAMED) alien bar to celebrate Valentine's Day, but a cute alien girl named Lyra (from the planet Starhaven) saves him from two thugs looking for a fight. After Winn mentions that he knows a bit about her planet's customs, Lyra asks him on a date.
Supergirl stops a bank robbery, but Mxy appears and tries to kill the robbers with their own guns. Supergirl blocks the bullets, but Mxy thinks it's just a part of the wooing process.
Supergirl fills the DEO on how dangerous Mxy is. Mon-El wants to kill Mxy, but Supergirl advocates a non-lethal approach. J'onn tells Winn to go through the DEO archives to find an alien artifact that can negate Mxy's powers.
Kara and Alex return to Kara's apartment to find it's full of roses. Kara also suggests that Alex turn Valentine's Day into a holiday that she and Maggie can celebrate. Just then, Parasite appears out of nowhere outside of her apartment building. Supergirl and Mon-El tries to fight him, but Mxy arrives (dressed in a Superman costume) and stops Parasite. Mon-El threatens Mxy, but Supergirl tells Mon-El to leave. Mxy warns Supergirl that he won't stop until she says "I do" and that things could get bad for Earth very soon.
Back at the DEO, Mon-El and Kara fight over how to handle Mxy. Mon-El claims that Kara is holding him up to a double standard, but Kara thinks that Mon-El is just weirdly jealous. Kara realizes that her and Mon-El were a bad idea. Later, Mon-El takes an artifact that Winn suggests might kill Mxy.
Maggie snaps at Alex when she tries to throw an impromptu Valentine's Day date for them. Maggie admits she hates Valentine's Day because it marked the day when she got kicked out of her parents' house.
Winn arranges for a nice Valentine's Day for his new alien friend, but she starts making out with him instead.
Supergirl finds out that Mon-El took the alien artifact to kill Mxy. Sure enough, Mon-El challenges Mxy to a Daxamite duel. Mxy uses the opportunity to transport them to a theater and make a ton of Hamilton references, but Mon-El gets the upper hand thanks to the artifact. Unfortunately, Mxy gets his hand on the artifact and teleports it away. Right before Mxy tries to shoot Mon-El, Supergirl arrives and agrees to marry Mxy, saving Mon-El. She says to meet him at the Fortress of Solitude the next day, so they can have a Kryptonian wedding.
The next morning, Mon-El tries to talk Kara out of marrying Mxy, but Kara says that it's the only way to stop Mxy from wreaking havoc on Earth. Plus, she and Mon-El are just too different to work.
Kara runs into Maggie while looking for Alex. Kara says that Valentine's Day was important to Alex and suggests that Maggie make some changes if she cares about Alex.
Supergirl flies to the Fortress of Solitude and tells Mxy she's not marrying him. She only sent him to the Fortress to minimize any damage he could do if he took the news poorly. Sure enough, Mxy takes the news poorly and animates the crystal statues of Superman's father to attack Supergirl.
Supergirl busts up the statue and says she's done with him. She sets the Fortress's atomic cauldron to explode and seals the Fortress to lock herself inside. Mxy can't stop her from killing herself, but she does eventually allow him to type in the code to stop the detonation. Mxy realizes afterwards that the code was his name spelled backwards, which is good enough to send him back to the Fifth Dimension. Before he leaves, Supergirl tells Mxy that he can't force someone to love, but can only let it find him.
Winn meets Lyra for an official first date and says that while he doesn't mind being in a relationship with an alien, he doesn't want to get hurt. Lyra agrees to take it slow and immediately demands Winn take her back to his place.
Maggie gives Winn a fancy dress and throws an impromptu Valentine's Day dance to make up for being an idiot for earlier.
Mon-El shows up at Kara's apartment and apologizes for acting like an ass earlier. Mon-El admits that Kara is his kryptonite and that he's never felt like this before. Mon-El tries to leave, but Kara says that she only said they weren't a romantic match earlier to protect him from Mxy. With Mxy gone, that leads to Mon-El and Kara finally kissing.