The three World Killers finally get their little coven going, while Supergirl goes on a dangerous mission to reach Sam. Also: Lena is in big trouble with the DEO.
Spoiler warning! The following is a complete recap of Supergirl Episode 317: "Trinity." To catch up, check out last week's recap here.
Lena finally admits to the team that she has known Sam is Reign, and has been hiding this secret for the last few weeks in the hopes of saving her. No one is particularly happy with this, and even less so when Lena admits she was keeping Reign at bay with Kryptonite from her brother's vault.
The three World Killers have met up at their fortress, and the Dark Kryptonian is telling them they must kill the humans within them. Pestilence kills Grace without a struggle, but Sam and Julia still hold on. The Dark Kryptonian believes this will come to pass, but until then, the three join together to start a total solar eclipse.
The solar eclipse causes Supergirl to briefly lose consciousness. While she is out, she is in the dark valley, and she sees Sam there. Previously, she had only seen Reign there. When she wakes, Supergirl tells her friends that the World Killers were created by Kryptonian witches. And she decides to fight fantasy with science fiction.
Brainy can send Supergirl back into the dark valley, and he agrees to send Alex and Lena with her as backup. The goal is to speak directly to Sam and figure out where the World Killers are hiding out. This could be especially dangerous since the eclipse is weakening Supergirl with every passing minute. Before they go in, Supergirl makes Mon-El promise that if things go sideways, he will pull out Lena and Alex, and let her finish the job.
In the dark valley, Grace is already dead, and Julia is slowly losing the battle for her sanity. She sees a little boy Purity killed, and hears the whispers of others she has killed. Sam, still sane for the moment, tries to bring Julia back from the brink, making her remember who she is, who her family is. They scratch these memories into the rock formations in the hope of securing their sanity.
Supergirl, Lena, and Alex arrive in the dark valley and set about looking for Sam and Julia. They are confronted by a Kryptonian demon that is pretty evil-looking, but nothing happens with it. It was kind of disappointing. Anyway, the girls explore, with Supergirl growing weaker by the minute. "Is this what it feels like when humans exercise?" Supergirl asks. "This is terrible. Why would anyone exercise?"
The girls finally find Sam and Julia, and both are falling down the rabbit hole of insanity. The scene gets a little Sisterhood of the Traveling Pants, and they eventually manage to clear Sam's head enough to send her back into the real world -- but that brings Reign down into the valley. And she is violent.
Up in the fortress, Sam breaks free of her sisters, who don't seem to notice. She wanders around, finds the crystal, and puts it into the machine, which pops up a location marker on Imra and Mon-El's map. With the fortress located, Brainy pulls the girls out of the valley, just as Reign is about to snap Lena's neck. In the fortress, Pestilence hits Sam in order to bring back Reign. It works -- just as Supergirl, Mon-El, Imra, J'onn, and Alex (in a spiffy new super-suit Winn made her) show up, looking for a fight. And they have found it.
It's an all-out fight. Supergirl only manages to hold off Reign until the total eclipse of the sun, at which point she loses all her powers. Despite being outnumbered, it seems that the World Killers will win. But then Supergirl and Alex manage to get through to Julia, who remembers what Reign said about only someone of equal strength being able to beat her. Julia uses her supersonic scream to knock Reign back, but Pestilence comes up and stabs Julia. She has enough time to shoot a supersonic scream at Pestilence, and two of the three World Killers are dead.
Or are they? A telltale white shimmer floats off of Julia's body; a matching green shimmer floats off of Pestilence. But then these shimmers take residence within Reign, just before she flies away. Weirdly, no one seems to notice this. Everyone is celebrating the defeat of two-thirds of the World Killers. With the World Killers no longer in the fortress, the sun returns, Supergirl's powers are back, and the fortress collapses.
During the big fight, James (as Guardian) goes into Lena's lab to check and make sure that she really, truly does not have any more Kryptonite. But when he gets there, he can't bring himself to betray her privacy. Instead, he assures Winn there is no Kryptonite left. Later, James admits what he did to Lena and apologizes. They hug it out (after all, a lot of secrets came out tonight), but Lena has one more secret to share. The Kryptonite wasn't Lex's; Lena has learned how to make it herself.
Also: Mon-El has not-so-subtly admitted to J'onn that he is more worried for Kara than he wants to admit. When the World Killers have been "defeated," Imra is overjoyed and excited to return home. Of course, Mon-El is not looking forward to that.
And now that she is super-powered, Reign has decided the only way to break free from Sam is to kill Ruby.