'Supergirl': This Is How Mon-El Can Be on Earth

Mon-El returned on tonight's Supergirl, and in addition to where he's been for the past seven months, the episode answered another important question about his return.

Spoilers for tonight's episode of Supergirl, "Wake Up," below.

On tonight's episode, Supergirl/Kara Danvers (Melissa Benoist,) J'onn (David Harewood,) and Winn (Jeremy Jordan) investigated the mysterious alien ship underneath National City and discovered a very much alive and well Mon-El (Chris Wood). They take him back to the DEO to get checked out, and Winn wants to know how it is that he is able to breathe since, last we saw Mon-El, Earth's atmosphere had been impregnated with lead and made toxic to Daxamites. While he doesn't answer then, we do find out before episode's end just how Mon-El is able to breathe on Earth -- and it's not a Legion of Super-Heroes flight ring.

As Kara quickly figures out, the Mon-El who has returned isn't exactly the same Mon-El she said goodbye to seven months ago, and after some drama -- including Mon-El stealing an alien artifact from the DEO to use on the ship and attempt to save the passengers in the cryo-stasis tubes -- we find out why. Mon-El didn't just disappear into space. A wormhole took him into the future, to 31st century Earth. Four hundred years into the future, L-Corp develops a cure that resolved the lead toxicity for Daxamites issue. It's kind of fitting considering it was an L-Corp device that caused the issue in the first place.

Of course, the cure isn't the only thing that Mon-El found in the future. For Mon-El, it hasn't been seven months, it's been seven years in Earth of the 31st century and the woman in one of the other cryo-stasis pods isn't just a teammate or friend. It's Imra Ardeen/Saturn Girl (Amy Jackson), and it turns out she's Mon-El's wife.

How this will impact Kara and the rest of the team going forward is anyone's guess, but one thing that's certain is that Kara won't have much time to process the news. She'll be busy fighting Nazi's in next week's Arrowverse crossover event, "Crisis on Earth-X", and maybe even kissing Oliver Queen.

Supergirl airs Monday nights at 8/7c on The CW.