“Dark Side of the Moon” is episode twenty of season three of Supergirl.
At the end of the previous episode, “The Fanatical”, more of the black rock was detected on an asteroid about 5 lightyears away. Supergirl and Mon-El are heading there in J’onn’s spaceship, but what they find is a city on a rock protected by a force field and they get hit by a tractor beam. Turning the ship into a rock gets them out of it.
At the DEO, Winn fills J’onn in on what he knows about Supergirl’s mission. And wants to know what J’onn hasn’t told him. J’onn says Reign is becoming immune to the synthetic Kryptonite faster than expected. Which is bad.
Alex is at home with Ruby; Alex reassures Ruby that Supergirl is going to what they need to fix Sam. And suggests they go out for ice-cream. Alex tries on some sunglasses, checking her reflection in a mirror. Or, more precisely, the person watching her. She gets Ruby out of the way and then she is shot at and a grenade thrown at her. Which Alex gets out of the way.
Lena is in her lab and Reign starts taunting her. The wall doesn’t work and nor does the Kryptonite. She’s getting stronger. Reign says that Lena needs Supergirl and she needs Lena, but when this is done, will they still need each other? Lena says they both agree that Reign needs to be stopped. Reign says the only way is to end her. And Sam.
Supergirl and Mon-El have landed; the black rock is in the city. They come under fire and flee, arriving in the city. The inhabitants seem human, but they need to change to fit in. Supergirl says she’s tired of hiding who she is; it’s exhausting. Mon-El says in the future, all the Legionnaires live openly. And it’s still exhausting.
Alex and Ruby arrive at the DEO. Once Ruby is out of the way, Alex says someone is trying to kill her. J’onn suggests maybe it’s personal and predates her time at the DEO. And it seems Ronald Collins, the former sheriff seen in “Midvale”, has just been released on bail. And he now lives six blocks from Alex’s apartment. Alex wants Winn to watch Ruby.
Supergirl and Mon-El track the rock to a building. With Kryptonian writing. They are surrounded by Kryptonian robots. Which are called off by Alura. Supergirl’s mother. Kara’s father created a shield to protect Argo City, but they weren’t sure it would work. Kara tells her mother they are looking for a black rock and traced its signature here. Because the Worldkillers came to Earth.
The shield that protects the city is powered by the black rock. They mined all they could, and every ounce means one more moment of life. It’s not Alura’s to give, so she’ll call a meeting of the High Council.
Winn doesn’t do so well with Ruby to start with. It doesn’t look like Collins is trying to kill Alex. Lena can still kill Reign, but not for much longer. And there may be issues in Argo.