This article contains spoilers for Supergirl season 3, episode 20, 'Dark Side of the Moon'.
Last night's episode had the main story that was teased in the trailer and a second story that, while not connect to the main story of the season, did address one of the sub-stories in a significant way. The main story focused on Kara (Melissa Benoist) and Mon-El (Chris Wood) flying out to an asteroid to find a chunk of the black rock Lena (Katie McGrath) needs to separate Sam (Odette Annable) from Reign. But when they get to the asteroid, they discover an entire city.
Meanwhile on Earth, Lena is struggling with the thought of what to do as Reign gets more powerful. Her using kryptonite to control her will only last so much longer, and she could flood her with kryptonite, but that would kill both Reign and Sam. And things aren't quiet at the DEO as someone tries to kill Alex (Chyler Leigh) when she's out in the park with Ruby (Emma Tremblay).
Focusing on Kara, they get into the city and start searching for the rock only to find out that it's Argo City, Supergirl's home on Krypton — and when they find the rock, they are also discovered by Kara's mother Alura (Erica Durance). She explains how after sending her off to Earth, Kara's father repurposed some of his experiments into making an energy field to protect the city from the destruction of the planet. He died in the process, but the black rock Kara needs is what keeps the city existing. They have to ask the city council to get what they need.
They are denied at first, but Kara makes an impassioned plea that wins over just enough of the council, including the dark Kryptonian (Anjali Jay) who has been helping Reign and the Worldkillers. By the time they get the rock, say their goodbyes, and get back to Earth, Reign has grown powerful enough to break free of Lena's cell. Supergirl and Mon-El get there just in time, with Supergirl jumping into fight Reign and Mon-El trying to get the black rock to Lena. That's where the episode ends.
The Alex story tied back into the Midvale episode as the Sheriff that Alex and Kara caught as teenager was out of prison and living in National City, but he turned out not to be the one trying to get her. So they set up a trap, with J'onn (David Harewood) posing as Alex and jogging in the park while a DEO team stands by. The guy trying to kill her attacks the real Alex instead and she gives chase, risking her life before taking down the guy and discovering it was the brother of a Fort Ross alien that she had captured.
Later, she is questioning what she did. How she could be the type of person that would jump off a roof without hesitation or fear of death in the line of duty and still except to be a mother with someone dependent on her? There is also a moment of bonding between Ruby and Winn (Jeremy Jordan) over them both having homicidal parents.
The episode was good from a main story standpoint, moving things along with trying to save Sam and with why the Worldkillers were sent to Earth. The fact that the woman helping them is actually from Argo City bring that part of the story together and makes the introduction of the city a big issue. The Alex storyline brought together elements from previous episodes, which I think the season needed as it was starting to feel a bit disconnected from the earlier part of the season. Like, where is Morgan Edge? It was nice to see Alex continuing the thoughts that lead her to break up with Maggie in the first place. And for the first time this season I got the impression that they might be able to save Sam. It felt like Ruby was going to end up with Alex at the end of the season to pay off the breakup. But now I think they've paid off part of that and shaped her future a bit more.
I'm not sure about that leap from the side of the building up and taking out the alien… did Winn put strength enhancers in her new suit or something? The whole thing including 'superhero' landing felt a bit more than we're used to seeing her capable of doing. But it looked cool.
Kind of surprised by the trailer for next week. To end the episode on the start of a battle and then show a trailer that talks about Supergirl leaving to go to Argo City, it seems. I haven't seen a bait and switch like that since Image Comics began, the missing fight between Youngblood #1 and #2. It was obvious this question would come up once Argo City and Alura were found, whether Kara would stay on Earth or go live with them. Just seems kind of quick to get to the question since Reign isn't even defeated yet.