A new villain comes to National City, putting Supergirl on high alert. Meanwhile, Winn's girlfriend, Lyra, gets Winn in trouble with the law. Maggie attempts to help Winn but old loyalties get in the way. The Music Meister attacks Supergirl.
Kara and Mon-El are relaxing at home watching Game of Thrones and musicals, and kiss. Meanwhile, aliens hack into National City's TV networks, demanding Earth turn over Mon-El of Daxam. The D.E.O. analyzes a spaceship in orbit. Meanwhile, Winn and Lyra are at the National City Art Gallery. Lyra bribes Winn with museum sex to get him to break into the art museum.
Supergirl flies up to confront the spaceship. The cruiser opens fire on Supergirl. She retaliates with heat vision, but it is stopped by the ship's shields. The ship then fires a force field bubble trapping Supergirl and sending her plummenting below. She breaks free just before the bubble crashes into the ground and flies back up. Before things escalate, Mon-El turns himself over to the invaders. Kara protests letting him go alone, but he assures her that he will be okay. As Mon-El is being beamed up, Kara joins him at the last second and the two of them arrive on the spaceship. The guards bow down to Mon-El and he reunites with his parents. Rhea and Lar Gand apologize to Supergirl for "defending" themselves, claiming that Supergirl was attacking them. Mon-El then introduces his parents as the Queen and King of Daxam to Kara and that he is the Prince. A shocked Kara is invited to dinner with the family.
While singing at the D.E.O., Winn is summoned by Maggie to the NCPD station. Maggie interrogates him about an art theft last night at the art gallery. Surveillance footage seems to frame him for stealing Starry Night since Lyra isn't in the camera footage.
At dinner, Rhea and Lar Gand talk about their quest to find Mon-El and how they heard of his exploits on Slaver's Moon. They ask Mon-El to come back to help restore Daxam. Kara asks how he really escaped Daxam's destruction. Mon-El tells them of how he woke up to the crisis, a palace guard ushered him outside and killed a Kryptonian emissary, and commandeered his ship for Mon-El to escape. Kara then leaves dinner, furious with Mon-El.
At the NCPD, Maggie is still questioning Winn when Alex and James arrive to the station. Maggie reveals that Lyra being a Valerian menas she has the ability to remain invisible to video cameras and that she used this ability to set Winn up. Alex asks for Winn to be released into her custody until they can find proof that Winn is innocent. At the D.E.O., they discover similar art theft cases and prepare to go look for Lyra, starting at the Alien Bar. Alex tells Kara to stay behind because she can tell that Kara is upset with Mon-El. Rhea then transports into the headquarters and asks to speak to Kara.
Rhea thanks Kara for looking after her son and to convince her to help get Mon-El to return to Daxam to lead their people. Rhea asks her to encourage Mon-El to speak to his parents.
At the trailer park, Winn confronts Lyra, who insists that she was conning him the whole time. Winn and Guardian attempt to arrest her, but two goons armed with laser guns attack them. Alex also arrives and helps Guardian fend off the goons while Winn goes after Lyra. They capture Lyra and the painting as the two get away. Winn goes to Lyra's cell. She tells him that the gang she's working for has her brother Bastian. She tells him of the war on Starhaven and that she and her brother were smuggled to Earth for safety. Bastian fell in with a gang but accrued a debt. Lyra agreed to steal Starry Night to pay off his debt and that if the painting wasn't delivered to Mandrax by tonight, that they would kill Bastian. J'onn prepares the D.E.O. to go after Mandrax and his gang but tells Winn that Lyra is still conning him. Winn, stilling believing in Lyra, frees her and they go after her brother.
Kara talks to Mon-El and tells him she talked with his mother. She is still upset with him and tells him to start acting like the prince and to go be there for his people.
Lyra and Winn meet up with Mandrax and his gang to give him the painting. Before they release Bastian, Mandrax discovers that the painting they gave him was a fake. He moves to kill them but Guardian jumps in front of him and punches him to no effect. They run, but Winn and Lyra are captured by the gangsters. Winn grabs one of their guns and points it at them in a standoff. Guardian continues to fend off Mandrax but is overpowered. Mandrax moves in for the kill, but the D.E.O. arrive and arrest everyone. Lyra is surprised by Winn's plan and then reunites with her brother.
At the police station, Winn gloats about taking down an art theft ring and being exonerated, but Maggie angrily tells him to leave. Winn says bye to Lyra and kisses her. Kara asks him why he still cares about her after she lied to him. Winn says that Lyra never meant to hurt him.
Kara arrives to her apartment, with Mon-El already being there. Mon-El admits to being the prince but that Kara had inspired him to be a hero. He says that he loves being with her, but Kara claims he only likes taking the easy route. She breaks up with him and shows him out the door.
Mon-El returns to the spaceship, but declines going back to Daxam. Rhea alleges the Kryptonian girl has poisoned Mon-El's mind. Mon-El says that she was the best thing to happen to him and tells them to leave Earth. Lar Gand allows his son to leave.
The next day at the D.E.O., Winn is admiring Cisco's interdimensional extrapolator device. J'onn annouces a new prisoner is arriving. The prisoner hypnotizes and knocks out Kara, frees himself from the handcuffs, and disables the guards, before using the extrapolator to escape to Earth One to chase down the fastest man alive. Kara wakes up in a dream world where she is being ushered on stage to sing in front of a crowd.
- The term star-crossed means "thwarted by a maligned star". It is used to describe lovers that have their relationship threatened by outside forces beyond their control. The episodes title refers to Kara and Mon-El being star-crossed lovers who have their positions of being from Krypton and Daxam, opposing planets, coming between them.
- Kara says "six seasons and winter isn't coming" which refers to TV show Game of Thrones.
- Also Kara mentions musical Funny Face with "Fred Astaire" and "Audrey Hepburn".
- When Lyra breaks the museum's circuit, Winn refers to her as a "She-Hulk", referring to the Marvel Comics character.
- After Lyra frames Winn, Kara claims that she will "Nancy Drew the crap out of" the situation, referring to the fictional detective.
- Winn calls Lyra a femme fatale, which Alex proceeds to tell him that he has a type. This refers to Winn's last relationship with Siobhan Smythe, who later became the super villain Silver Banshee.
- The alien that Alex interrogates asks for Hamilton tickets. Alex agrees, reasoning that the actor who plays King George is an alien from Rimbor Five, owing her a favor. At the time of the episode's airing, the actor portraying the Broadway version of King George II was Taran Killam.
- When Music Meister first sees Kara, he says, "Oh, there you are. I've been looking for you," which is also the same line Darren Criss's character Blaine says on Glee when he confesses his feelings for Kurt. Coincidentally, it was also on the sixteenth episode of the second season.
- At the end of the episode, a card that says "To be continued on The Flash" is seen. This is the same card used in "", which was part of the four-night crossover, , and was followed by episode "".
- The episode's plot bears some similarity to the transition arc between the third and fourth seasons of Lois and Clark, where two Kryptonians arrive on Earth in order to take Kal-El away so that he can serve as the ruler of a surviving colony.
- Lar Gand refers to the debris from Krypton's explosion that enveloped Daxam as Kryponite; a term supposedly coined by J'onn on Earth some years after the cataclysmic event.
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