There are a lot of big reveals in this week's episode. We learn who the Dark Kryptonian is, and we meet another important character. Because of this, major spoilers follow!
The following recap is for Supergirl Episode 320: "Dark Side of the Moon." It contains major spoilers. Catch up with last week's recap here.
Mon-El and Kara head off on their intergalactic road trip. They get sucked into a tractor beam and pulled down onto this tiny planet. The Rock of Yuda Kal is located about a mile underneath the city they see in the distance. They set out, are fired upon, and duck into a bazaar. They steal some cloaks to disguise themselves (during which time Kara makes a vague attempt to contemplate her distaste for always needing to disguise her true self) and continue looking for the Rock. They finally find it, in what Kara identifies as a Kryptonian memorial. Then they are beset by droids. The droids are sent away and Kara gets a huge surprise: Her mother is there, in the flesh. It is not a hallucination, nor is it a hologram. Alura Zor-El is alive.
The women hug and fill each other in on what happened. Alura explains that Kara's father created a shield to protect the city of Argo. They sent Kara away because they didn't expect it to work. But it did. After the destruction of Krypton, the young and elderly got sick from the fallout. They discovered that the Rock of Yuda Kal cleaned the atmosphere and made the city livable. The shield hides them, but it prevents radio signals from entering or escaping. After Kara explains the World Killers, Alura promises to call a meeting of the council to take a vote on whether to gift her a piece of the Rock.
Kara goes before the council, and we learn that the Dark Kryptonian, the one who controls the World Killers, is the head of the council. Kara pleads her case, and the vote is split: three to give Kara some of the rock, three to send her away empty-handed. The Dark Kryptonian is the tie-breaking vote... and she votes to give Kara what she asks for. Is it a trick? We don't know yet, but as soon as Kara and Mon-El leave Argo, she dons her evil cloak and calls to Reign. Back in the L Corp lab, Reign taunts Lena, trying to goad her into killing her. Supergirl and Mon-El show up at the last possible moment, and Reign is ready for a fight.
Meanwhile, Alex is doing her best to keep Ruby physically and emotionally healthy, but things get a little out of hand when an attempt is made on Alex's life while in the park with Ruby. They get back to the DEO in one piece, and Alex sends Ruby off to read. As soon as she has a moment alone with J'onn, she reveals that this person was definitely trying to kill her. After going through a list of suspects, Alex decides it was probably the first arrest she made, before she joined the DEO, a sheriff who killed a kid. It turns out not to be the sheriff, and after setting up a trap that backfires and leads Alex on a dangerous rooftop run, she discovers the killer is the twin brother of an alien she had arrested previously. It was an involved subplot that ultimately had little payoff.
- Mon-El, while on Argo, gives a young, ill boy a vaccine patch that, when left on for a week, should cure him of his lung disease. He tells him that when he is done he should give it to someone else who can use it (which just sounds unsanitary).
- After getting into a fight with Ruby, Winn finally relates to her on a human level, and they eventually become buddies.
- Alex stresses having to choose between being a mother and keeping the job she loves. J'onn promises she will know the right choice when the time comes. In other words, he offers her comforting, noncommittal platitudes.
- Lena is getting nervous waiting for Supergirl to return with the Rock. She confides in James that she may have to make a horrifying choice: to overdose Reign with Kryptonite. It would kill Reign -- and Sam -- but it would protect the world.