The White Martians are invading Supergirl on Monday, and it will bring up a lot of bad memories for J'onn J'onzz (David Harewood). The episode will reveal that the White Martians oversaw a genocide of J'onn's people: the Green Martians. Confronting his past will only put more pressure on J'onn's alter-ego, Hank Henshaw, as he tries to keep calm and continue running the DEO without anyone else discovering his true alien identity.

Their arrival also means we'll find out how J'onn J'onzz became stranded on Earth and why he decided to be Earth's defender from alien attacks. TVGuide.com got the skinny from Harewood:

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How much will these White Martians align with the White Martians fans know from the comics?

David Harewood: They look very similar to how they look in the comic books, but they also have the ability to shape-shift. What you see on television is probably a frightening depiction of what they can be. It's definitely going to be a surprise.

How will this episode reinforce what Hank has been saying about not being able to be his true self on Earth and make Kara [Melissa Benoist] see the danger in it?

Harewood: People aren't going to take too kindly to an alien impersonating a human being and running a secret government organization. Not only is he in trouble because he's not who he says he is, but he's also an alien at the heart of this government organization. If he's not careful, this will not end well. It's okay for Kara to say, "It's all right. Come out and be who you really are," but if Hank does come out as who he really is, heads are going to roll. Everything is going to be a little too hot and [there will be] too many questions for it to go smoothly.

How will this White Martian appearance bring him and Kara closer together?

Harewood: It is the beginnings of a little family. J'onn J'onnz had two daughters that he lost. [Alex and Kara] lost a father, so in a way each of them becomes his daughters. He's a father figure to them. We see them move forward. It's family bond. It's not just boss and worker.

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Is J'onn's ideal situation to be himself on Earth or does he have hopes of returning home to Mars one day?

Harewood: He's very much aware that he's the last son of Mars. He would like to go back and see his planet. He hasn't been back in many years. At some point during the Justice League comics I was reading, he does go back and find his brother. There's trajectory and room for that, but as Hank Henshaw, his main goal is to protect Earth from alien attack. He's, almost by accident, found his true vocation. He's sworn to protect these two girls and he's also sworn to protect Earth. He will do anything within his powers to do that. If that means impersonating the head of the DEO, what better guise to come to him?

Outside of the White Martian, Maxwell Lord (Peter Facinelli) and General Lane (Glenn Morshower) are also antagonist characters to the overall Supergirl mission. Who is the biggest threat right now?

Harewood: We're surrounded by threats. There are literally threats coming from all directions. We are walking a very tight line, particularly because we have two aliens working for this organization. It is going to be fraught with danger and possibilities. In the latter half of the season, we really do see tensions and situations ramp up.

Supergirl airs Monday at 8/7c on CBS.

(Full disclosure: TVGuide.com is owned by CBS).