Kara Danvers was successful in taking back Earth from Rhea and her alien armies but she lost someone close to her in the Supergirl season 2 finale.
In episode 22 Nevertheless, She Persisted the Girl of Steel challenged the Queen of Daxam to a time-honoured fight to the death with the winner getting planet Earth. But when Rhea betrayed their pact, Kara was forced to resort to her backup plan, which threatened her future with Mon-El.
Lena Luthor, with help from mother Lilian and Winn, recalibrated a device, created by her evil brother Lex, and filled the atmosphere with lead, which is toxic to Daxamites. While it took care of the city's Daxamities problem, as they all fled and Rhea turned to dust, it also meant that Mon-El would have to leave the planet.
Mon-El tells Kara in a tearful goodbye scene, "Wherever I go, I'm going to be better because of you. You'll be in my heart." She replied, "You've made me so happy," and gave him her necklace, finally acknowledging that she loves him. "I should have said it before."
They both share an embrace as Mon-El leaves Earth in a pod, leaving a crying Kara behind. After a long talk with Cat Grant, Supergirl flies, determined to find her love, as Mon-El's pod is seen getting knocked off course and pulled into what looks like a Phantom Zone wormhole, which sounds familiar to comic book fans.
In the comics, Mon-El, suffering from lead poisoning, is placed in the Phantom Zone to stop him from dying and is eventually released by the Legion of Superheroes in the 30th century once they found a cure.
Melissa Benoist, who plays Kara, had already apologised to fans in advance for the events of the finale. She told Comicbook, "I think it's a huge finale, and there's a lot going on, and I think people will go on an emotional roller coaster with Kara, so I'm sorry to all the fans in advance."
"Every time I get a script, I'm like, 'There's no way we're going to top what we just did.' And then I read the new script and I'm like, 'how are we going to do this?' And somehow we do. So I think we'll only just keep getting bigger and bigger. The scope of this world is so massive that we can't go back," the Glee actress added of this Monday's finale.
Will Mon-El return in Supergirl season 3? We will know more when the show returns in the fall.