SPOILERS for Supergirl Season 3, Episode 18: "Shelter From the Storm."
James and Lena are attacked by Reign after spending the night together. Reign is looking for Ruby. Lena only scares her away after hitting her with a spray of kryptonite aerosol. The team convenes and plans their action: Supergirl and J'onn will go to keep Sam's mom, Patricia, safe. Winn will rig up an electricity harness. Alex is going to check in on Ruby — but only after she drags Ruby's location out from Lena. Lena admits Ruby is hidden away in Lex's mansion. It is 5,000 steps end to end, doesn't appear on any map or satellite, and has an invisible mode. Lex may have had the same contractor as Batman.
Let's start with Supergirl and J'onn. They go to protect Patricia, who admits she never told Sam where she comes from because she wanted her to be "normal." Patricia was an awful mother who kicked out her daughter and has never met her granddaughter. If Sam/Reign is coming here, Patricia wants to be there. She should have helped her; she's not going to turn away now. They don't have to wait long. The potted plants start to die (apparently Supergirl still hasn't figured out that Reign now has the other World Killers' powers) as Reign comes up the driveway. She calls for Ruby, but instead finds J'onn and Supergirl. A battle ensues, which includes sonic screams, laser eyebeams, and throwing people through walls. Reign ends up stabbing Patricia, but weirdly, she doesn't die there. She survives until they arrive at the DEO, where she tells Supergirl to tell Sam she is sorry and Ruby that she loves her — then she dies. It was a completely useless scene that I can only imagine was inserted for time.
Frustrated, Supergirl and J'onn go visit M'yrnn, who has been depressed as his dementia worsens. M'yrnn once managed to talk down soulless white Martians, but he can't remember how. J'onn transforms into a white Martian to scare him into remembering. It all becomes clear: She just has to focus on Reign's twisted sense of justice. Back at the DEO, at James' insistence, Lena reveals to Supergirl that she had figured out how to make Kryptonite and brought some that had been super-charged to use against Reign. Supergirl is horribly offended by this, takes it as a personal attack, and basically blows off Lena.
Before Brainy, Imra, and Mon-El headed back to the future (complete with hackneyed Back to the Future jokes), Brainy gives Winn a gift: 31st-century dirt, which Winn is worryingly excited about. What he didn't tell him is that hidden in the dirt is a bugging device. So Brainy now knows that Supergirl and her friends are facing Reign with Purity and Pestilence's powers. He also says that their chances of beating Reign are 51%, but the chances shoot up to 88% if just one Legionnaire helps. Mon-El wants to help, and just before it is too late for him to return, Imra gives him permission. She knows he is struggling with his emotions for Kara, and wants him to go back and figure out if he wants to stay with Kara or return to Imra. Either way, she seems pretty chill about it.
Alex pays Ruby a visit, but she puts on a sunny disposition and won't tell Ruby anything. Ruby is obviously frustrated, so she sneaks outside with Alex's cell phone and tries to call her mom. Sam doesn't answer, but Reign, who has been hovering over the city, hears Ruby and makes a bee-line to Lex's mansion. All she has to do is break the control panel and Lex's invisible mansion is suddenly visible (seems like a tech genius could have made that a little more difficult). An alarm goes off, and Alex sends Ruby to hide in the basement vault.
The vault is the ultimate safe room. Reign must make it past a hallway of fire, liquid nitrogen, and bullets — which she does. It only takes three hits to break down the door, at which point a massive machine gun starts firing. The bullets bounce off Reign. Supergirl and Mon-El show up and another big battle scene ensues. Mon-El has Lena's Kryptonite, and while Supergirl is getting her ass handed to her by Reign, Mon-El forms the Kryptonite into a bullet, feeds it into the machine gun, and fires. This weakens Reign enough that Alex is able to shoot her with the electric net. Reign is taken down. During the fight, Reign's mask comes off, and Ruby sees her mom is Reign. Those are going to be some expensive therapy bills.
Reign is taken back to Lena's lab, where she sets to work trying to save Sam. Supergirl apologizes to Lena for being weird about the Kryptonite, but it doesn't seem to change things between the two. Later, Kara runs into Lena on her way to see Ruby. Lena is glad to see Kara and fills her in on what has been going on — including that Supergirl isn't "truth and justice" like she says. Kara feels bad when Lena admits she can never trust Supergirl again.
There is a weird final scene that comes without any explanation or transition. A girl sneaks in and steals a prayer book from the Cult of Rao. That's it; nothing else. I'm sure it will be necessary in next week's episode, but it was so inelegantly jammed in this week.