Why Mon-El's Return On 'Supergirl' Won't Heal Kara's Heart

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Break-ups are hard no matter what the circumstances are, but when you have to send your alien boyfriend away because the planet's atmosphere will kill him? That kind of heartbreak is pretty major, and lasts a good long while. Supergirl's Kara (Melissa Benoist) has been learning that tragic lesson all season after Mon-El (Chris Wood) was forced to leave Earth in the season two finale last spring when the lead bomb irradiated the planet's atmosphere with his one weakness. In the wake of his absence, first Kara tried giving up her humanity entirely before slowly starting to integrate back into her friends and loved ones' lives. And though she may seem to be getting back to the girl she was before, she's still not over losing the man she loved.

And even though Mon-El is returning to Supergirl in this week's episode (airing Monday at 8 p.m. on The CW), things won't be sunshine and rainbows and romantic music swelling in the background. Fans remember that Mon-El's pod was sucked into a wormhole after leaving Earth, and comic book fans correctly guessed that the ship that has been woken up underneath National City is the Legion of Super-Heroes, the heroes from the 31st century including Saturn Girl (Amy Jackson) and Brainiac 5 (Jesse Rath).

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So does this mean that Mon-El has fulfilled his comic book destiny and joined up with the Legion? How long has it been for him since leaving Earth if he joined up with time-travelers? Will he be the same hero Kara said goodbye to last year? There are so many questions that Bustle headed up to the Vancouver set of Supergirl to get some answers.

"In Episode 7, it's pretty mysterious," Benoist tells Bustle along with a small group of reporters of Mon-El's return. "It won't be the reunion you expect. I don't want to tease too much, but the fact that the Legion is coming to the show has such a hold in the comic book world, especially with Superman [Tyler Hoechlin], Martian Manhunter [David Harewood], specifically. That's really exciting for us. For Kara, it's a brand new set of problems that you'll see pretty quickly."

While Kara might be optimistic about Mon-El's return, her sister Alex (Chyler Leigh) provides some much-needed "skepticism" about the circumstances of his sudden reappearance.

"After all that time, you can really figure out and understand what happened with Mon-El and the time that's passed and what that looks like," Leigh says, sitting on the alien dive bar set earlier this month. "It's not an easy pill to swallow no matter what, and for Alex, Alex is always going to be protective of her sister. They're trying to really wrap their minds around what that looks like."

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Leigh teases that "with the Legion coming in, it's like you never have a down moment or a dull moment."

"There's not a lot of time for dwelling, but at the same time, [Kara] is now constantly reminded of what happened, of that loss, that heartbreak," Leigh says. And Katie McGrath, who plays Kara's friend Lena Luthor, warns that Mon-El's mysterious return will have far-reaching ramifications.

"Kara is her best friend and the love of Kara's life has just come back," she says. "So that is going to impact on everyone, I think."

Harewood teases with a smile that the Legion ship "has some very valuable members onboard, so that should fun," which is especially true if Supergirl follows the comic book source material for Brainiac 5 when he was mentally unstable and became so obsessed with Supergirl that he made a robot in her likeness.

"We've only just introduced the two of them," Benoist notes of Brainiac 5 and Kara. "What we're working on now, I can't really tease anything. I think my lips have to be sealed on that one." Sounds like there might be some wildly unstable love in the air for a certain Legionnaire!