Lo, here be spoilers for Supergirl Season 3, Episode 10: "Legion of Super-Heroes."
Supergirl survived her encounter with Reign but is in a coma. Mon-El sends Braniac 5 -- or "Brainy," as he is called -- into Kara's brain to make sure there is no brain damage. Everything seems to be healthy, but Kara cannot leave her cozy loft, which is where her brain retreated for the coma. So now she and Brainy have to figure out what is keeping her there. The simple answer: fear.
The majority of the episode, luckily, takes place outside of Kara's brain, where the DEO team have to figure out how to handle the Reign situation. J'onn decides it is time to invoke Sundown Protocol, developed in case Superman ever turned against them.
Reign believes that Supergirl is dead, which leaves her ready to "deliver the awakening." She explodes into CatCo and, in front of a camera, tells National City that apathy is the greatest sin of all. Not only is she after the criminals, she is going to kill people who don't help when they see wrongdoings. Or, I guess, don't help fast enough, as one example is that Reign blows up a meth lab, and then when the cops arrive, she kills them, too. I guess for not stopping the meth lab before it was set up ... ?
While Sundown Protocol is being set up, Mon-El and Imra, after much hemming and hawing, finally reveal why they can't help. A plague hits farming planets that they call the Blight. It leaves planets dried-out husks. Mon-El and Imra have the info on how to defeat the Blight hidden in their DNA. If they die, their DNA doesn't make it to the future, and they can't save the universe.
The DEO agents stage a bank robbery to lure out Reign, who is only too happy to oblige. They hit her with a red Kryptonite bomb and all sorts of other specialized weapons. This seems to weaken Reign, but ultimately she escapes. Alex suggests that Reign was weakened by the Kryptonite; maybe she just needs a more potent, direct dose. They decide she must be injected, but now they have to find someone strong enough to break Reign's skin.
Luckily, Imra convinces Mon-El that now is the time for them to help. They suit up, with Brainy following in their ship, and they follow the destruction toward Reign. She hits Albatross Bay prison and moves through, gleefully killing anyone who gets in her way. The prisoners all try to act tough, but as soon as Reign reveals her power, they all try to get out of her way. All except Coville, who is praying in his cell. Reign recognizes the prayer, and he insists she doesn't have to spare him; just let him serve her. Bon Jovi blasts through the prison before she can make a decision, and she goes to the prison yard to meet Mon-El, Imra, and Brainy in the ship.
Reign seems a bit confused, but holds the trio off. When Mon-El runs at her with the Kryptonite injection, she seals him in a ball of ice. Mon-El, for whatever reason, decided to run like a human instead of a Daxamite. J'onn pops up for an assist, but it doesn't slow down Reign much. Luckily, it is around this time that Kara figures out how to leave her apartment (she was thinking like Supergirl; she had to think like Kara Danvers and use a key) and therefore wakes up. Despite Alex's protests, Supergirl has to go help.
Reign is more than a little surprised when Supergirl shows up -- she thought she was dead. Supergirl attacks, and even though the Kryptonite has weakened her, she manages to get the injection into Reign's bloodstream. Severely weakened, Reign flies off and hides in her fortress of solitude. Her Yoda appears and Reign admits that Supergirl had friends she didn't expect. That is when her mentor tells Reign there are "others like you." She just didn't tell her because she wanted Reign to prove herself. Now Reign is ready to figure out how to find them. Oh, and by the way, Coville is there, and of course he volunteers to help.
If I were Reign, I'd be pissed at her Yoda-hologram-lady. "I wanted to wait until you proved yourself before I would help you." In all likelihood, Reign would have either "proved herself," won, and wouldn't need any help, or would be dead. Anyway, you can bet that Yoda isn't going to tell Reign where to find the "others."