“Trinity” is episode seventeen of season three of Supergirl.
In the previous episode, “Of Two Minds”, Purity and Pestilence attacked Lena’s lab and took Sam.
Lena is now being interviewed at the DOE. She explains she’d suspected something about Sam and took her to an L Corp lab to run tests and discovered she definitely was Reign. J’onn asks if she didn’t think to bring it to their attention. No. She was working on isolating what turns Sam into Reign but didn’t manage to finish. The Worldkillers have now gone to ground. When asked, Lena says she kept Reign contained with Kryptonite Lex had kept in a vault.
The Worldkillers are being told that the humanity within them must completely die. Pestilence says Grace gave up without a fight. The other two are told that Sam and Julia won’t last long now and they will bring blessed darkness to the Earth so it can be reborn.
Sam is in the alternate world and she sees Grace’s body and the living Julia. Julia is seeing one of her victims and says they are all coming. Sam tells her it isn’t safe and they need to head to a Fortress in the distance.
Lena calls James at work to tell him that Sam is Reign and that Supergirl knows. And that Lena was working out how to free Sam and was using Kryptonite. She’s telling him because she gambled everything and failed.
Supergirl speaks to Lena afterwards about the Kryptonite. Lena says she used the last of it. Then the sun starts being eclipsed and Supergirl collapses. She sees the other realm and Sam and Julia before coming around in the DEO.
Alex, Supergirl and Lena discuss the eclipse which shouldn’t be happening. Lena doesn’t want to hear it’s being done through magic. Supergirl tells her it’s dark magic. The Worldkillers were created to do one thing; kill the world. There are no apocalyptic writings in the Book of Rao, but there is a mention about witches joining hands to bring blessed darkness.
In two hours, it will be totality and the temperature will drop. Supergirl says the Kryptonian valley she saw is the same one she’s been dreaming about for some time, but today she saw Sam, not the Worldkillers. Lena explains that it’s not a dream; it’s an alternate dimension where Sam goes when Reign takes over. If Sam could be woken, she could tell them where the Worldkillers are.
They want to use Brainy’s technology to enter the valley. He recaps what they want. It sounds like fun. Both Lena and Alex want to come with Supergirl to help, especially as Supergirl will be without her powers there.
Julia is freaking out about the people she killed and Sam is reassuring her. Then Sam starts forgetting things, like Ruby’s name, and tells Julia they have to write down everything they remember.
Supergirl heads to see James; she’s concerned about the Kryptonite and wants Guardian to break into Lena’s lab to confirm there is no more.
Alex asks Lena why she didn’t tell the truth about Sam. For the same reason Alex didn’t tell Lena she was DEO. Of course Lena knew; her mother is number one on the DEO’s most wanted list. But it was Alex’s secret to tell, just as Sam’s is hers. And Ruby is safe. Supergirl returns; Brainy warns her two more people will weaken her.
Supergirl speaks to Mon-El privately. She wants the other two pulled out if things go wrong, not her.
All three are heading to a potentially dangerous alternate realm to try and get Sam to wake up so she can broadcast their attention, so the Worldkillers can be found and stopped before the eclipse causes too much damage.