“I’m on the brink of having everything I ever wanted — family, friends, a job that I love. A life as a hero I never could have imagined. And Mon-El,” Supergirl (Melissa Benoist) confesses to her cousin Superman (Tyler Hoechlin) in "Nevertheless, She Persisted," the season 2 finale of Supergirl. Alas, the Girl of Steel can't have everything, and by the conclusion of the episode, Supergirl and Mon-El (Chris Wood) were forced to say their goodbyes.
The future of National City and of Earth was dire. Queen Rhea of Daxam (Teri Hatcher), Mon-El's mother, had tricked Lena Luthor (Katie McGrath) into helping her build a portal that would allow the Daxamite fleet to travel to Earth. Christening the planet "New Daxam," Rhea launched a full scale invasion of National City, at one point even poisoning Superman with Silver Kryptonite in an attempt to get the Man of Steel to destroy his Super cousin, which ended in failure when Supergirl bested Superman in battle. Nevertheless, Rhea persisted in her invasion attempt. She was no longer interested in converting her son Mon-El back to her cause to rule Daxam as the crown prince alongside Lena Luthor. Rhea wanted Earth's blood laid upon Supergirl's hands.
Supergirl, Superman, and Alex Danvers (Chyler Leigh) came up with one strategy to bring Rhea down: Superman scanned the Fortress of Solitude's archives about Daxam and discovered the ancient Daxamite custom of Dakkam-Ur - a gladiatorial fight to the death. Supergirl made the challenge to mortal combat to Queen Rhea, and the Queen accepted, keeping the secret that her blood has been irradiated by Green Kryptonite since the explosion of Krypton that devastated neighboring Daxam. Rhea knew this gave her the edge she'd need to kill the more powerful Supergirl.
However, Lillian Luthor (Brenda Strong), the nefarious head of the alien-hunting cadre Cadmus, had her own plan: she brought a device from her son Lex's vaults to Lena. This device, once intended to be used against Superman, was meant to irradiate Earth's atmosphere with trace amounts of Kryptonite. Lena, with Winn Schott (Jeremy Jordan)'s help, was able to convert the device to emit trace amounts of lead into the atmosphere instead. This would render Earth lethally toxic to Daxamites - including Mon-El (though his months of exposure to Earth's yellow sun would allow him a few minutes more tolerance to the lead in the air).
After a season where Mon-El's heroic journey of self-discovery eventually forced him to grow from a self-involved frat boy from outer space into the kind of person who is worthy to be both a hero in his own right and deserve the heart of Supergirl, Mon-El was faced with an impossible choice - give up Kara and his life on Earth and flee or die with the rest of his people. Upon learning the Luthors' device might be the only way to stop his people invading the Earth, Mon-El did not hesitate for a moment to okay the use of the device, knowing full well the cost of either scenario. Still, he bet on his girlfriend to beat his mother in Dakkam-Ur, serving as Supergirl's second in the fight, until Rhea quickly broke their rules of engagement and ramped up the invasion. Mon-El heroically acted, taking to the streets of National City, fighting off Daxamite soldiers and saving as many innocent human lives as he could, while Supergirl finished the fight with Queen Rhea.
The Luthors completed the conversion of their device and Lillian quickly used it to repel the invasion. The atmosphere of Earth was infused with lead, forcing the Daxamites to immediately evacuate the Earth while Rhea died of lead poisoning. Mon-El's fate was sealed, and so was any future he might have had with Kara on Earth. Supergirl immediately gifted Mon-El with the Kryptonian ship she arrived on Earth in, and after a tearful, heartbreaking goodbye, Mon-El left Earth seemingly forever. However, while traveling in space, Mon-El was terrified when his ship was caught in some sort of wormhole and he disappeared.
Mon-El's whereabouts are a mystery Supergirl season 3 will surely address, as Chris Wood has grown into a valuable and endearing addition to the cast. That Mon-El didn't die in the finale is notable; it indicates Mon-El's story is far from over and a reunion with Kara is something the two heroes and the fans can look forward to. However, the question remains: Where did Mon-El go when that wormhole swallowed him and his ship up?
Did Mon-El return to Daxam? It was revealed that Daxam is habitable once more, and with King Lar Gand (Kevin Sorbo) and Queen Rhea dead, Mon-El's duty as crown prince would be to rule his homeworld. However, this is a duty Mon-El already actively rejected, and it's likely his own people wouldn't want him to rule them. It's also possible Mon-El was banished to the Phantom Zone. Kara herself was a captive of the Phantom Zone during her journey to Earth, and the Phantom Zone is a useful device Supergirl's writers can use to keep Mon-El "on the shelf" until they need him again in season 3. There's another enticing possibility, however, that draws straight from DC Comics:
Mon-El could have traveled to the future - specifically, to the 31st century. In the comics, Mon-El (who is also known by the superhero code name Valor) was once suspended in the Phantom Zone and freed in the 30th century to join in the adventures of the Legion of Superheroes - the expansive team of superpowered teenagers from many worlds who protected the galaxy. If Mon-El traveled to the future, this could be a way for Supergirl to introduce the Legion of Superheroes into the continuity of Earth-38. The Legion is already no stranger to The CW - Smallville featured members of the Legion of Superheroes in season 8, and one important seed to the coming of the Legion has already been planted on Supergirl: President Olivia Marsden (Lynda Carter) was revealed to be a Durlan. The Durlans are a shapeshifting race, and a Durlan helped found the Legion of Superheroes in the future.
Whether or not Mon-El will be sporting a Legion Flight Ring when next we see him, Supergirl's fans have a reunion between Kara and Mon-El to eventually look forward to, along with whatever new challenges await the Girl of Steel when she returns to The CW this fall for season 3.