SPOILER WARNING: This article contains major spoilers for "Crisis on Earth-X, Part 2," the second episode of this year's Arrowverse crossover.
The second part of "Crisis on Earth-X," this year's Arrowverse crossover, has revealed why Earth-53's Nazi forces have invaded Earth-1: they need Supergirl's heart. At the end of "Part 2," Overgirl collapsed where she stood, revealing that she was gravely ill -- and their intended cure is an involuntary organ donation from Supergirl.
As the Dark Archer, Oliver Queen's Earth-53 doppelganger, congratulated his team for capturing the Arrowverse's heroes, Overgirl fell to her knee with a gasp of, "The pain! I don't know how much longer I can bear it!" Dark Archer also went to his knees to comfort his ailing wife, saying, "It's alright. You're going to be alright, because we found her. We found her, and now we have her."
With that, the two evil doppelgangers looked over at Supergirl. "I love you. Now very soon, this nightmare will be over and I'll take you home. So stand," Dark Archer said, and Overgirl rose back to her feet.
"We need your Kara's cooperation," Dark Archer said to said to his captives. "Well, we don't need it, but it'd be preferable. But for the time being, you are more valuable as leverage."
"What do you want my sister for?" Alex Danvers demanded, only for Overgirl to respond, "To save my life." Turning back to her doppelganger, Overgirl continued, "That's your little mission statement, isn't it? Help people. Save lives. Well, now's your chance."
"The General is dying," Dark Flash explained.
"Her blood. She's been exposed to too much solar radiation," Alex said breathlessly. It was, in fact, this excess solar radiation that allowed Alex to track Overgirl back to their base in the first place.
"Yes. Like Icarus, I flew too close to the sun," Overgirl confirmed. "But we can save her," Dark Archer added. "We're going to save her. All we need is a new heart, and luckily, on this planet, we found the perfect donor."
"That's why you stole the prison," Barry realized. "So you could duplicate a red sun, weaken Kara and cut into her."
"Can't make an omelette with invulnerable eggs," Overgirl responded with a smile.
The Earth-X villains then brought the captured heroes -- Green Arrow, The Flash, Supergirl, White Canary, Alex and Firestorm -- to their world, where they were dropped off in a concentration camp. Because of the dog collars they wear, each hero was rendered powerless and unable to escape.
This year’s four-part crossover “Crisis on Earth-X” will air across two nights, beginning Monday, Nov. 27 with Supergirl and Arrow (on a special night for one week only) and concluding Tuesday, Nov. 28 with The Flash and Legends of Tomorrow.