This article contains spoilers for Supergirl season 3, episode 17, 'Trinity'.
The episode starts with the fallout from last week's reveal that Sam (Odette Annable) is actually Reign and that Lena (Katie McGrath) has known for three weeks and was trying to cure her. Supergirl (Melissa Benoist), J'onn (David Harewood), and the rest question her. Lena then catches James (Mehcad Brooks) up on everything she's done, figuring she'd lose him too but wanting to be the one who told him.
Meanwhile, Reign, Purity (Kris Marshall), and Pestilence (Angela Zhou) — under the guidance of the Dark Kryptonian (Anjali Jay) — begin the ritual to bring the darkness, covering over the sun. But Sam is awake in the alternate reality, finds Grace's body and then Julia, and tries to help her and look for safety. The ritual knocks Supergirl out for a moment and she ends up in the same place as Sam and Julia. When she snaps out of it, she goes to Brainiac 5 (Jesse Rath) and asks him to project her mind into that same valley so she can try to help Sam and Julia take control again. Lena and Alex (Chyler Leigh) insist on going with her.
Before she goes in, Supergirl flies to see James and asks him to break into Lena's lab to see if she has any kryptonite left (she was using it to control Reign and said it was all gone). He's hesitant but agrees. She then goes back and goes into the alternate reality after getting Mon-El (Chris Wood) to promise not to pull her out until the job's done. Oh, and she's getting weaker as the sun gets more covered, so the trip could kill her. He agrees.
Once inside they go looking for Sam and Julia. Lena calls Supergirl on not revealing who she really is. James, as Guardian, goes into L Corp with Winn (Jeremy Jordan) guiding him. He gets to the vault door but doesn't open it, choosing to not violate the trust of the woman he loves over verifying the safety of his friends. Inside the valley they find Sam and convince her to wake up and let them know where the Worldkillers are. They use the memory of Ruby to do it and she wakes up, still dressed as Reign, and figures out to put a crystal in a slot. That makes them visible to the Legion ship.
Inside the valley Reign is now awake there and attacks Lena. Supergirl offers to exchange herself for her friend, but Reign doesn't care. Then Mon-El, after having a touching moment with J'onn showing he still loves Kara, orders Brainiac 5 to wake them and they get back to safety.
Breaking the circle with the other two Worldkillers ended the ritual and started putting the sun back to normal. Pestilence slaps Sam to force Reign back to the surface, but before they can start the ritual again, the Legion ship arrives. Winn gives Alex a new suit with a few new tricks, like what appears to be a red sun gun.
Alex, Supergirl, J'onn, Mon-El and Saturn Girl (Amy Jackson) drop into the base and take on the Worldkillers. Martian Manhunter vs. Purity is a fun match. Pestilence vs. Mon-El and Saturn Girl is a lot of CGI flying an tossing around. Supergirl vs. Reign with Alex helping is also good. Eventually Alex gets through to Julia to take back over and she uses her powers against Reign and then fights Pestilence. Purity and Pestilence kill each other with their powers going into Reign… who then flies away, and the fortress collapses.
The episode wraps up with Supergirl telling Lena that they're okay. Imra talks to Mon-El about how they've done it — they saved the future, and now they can go home. Well, it's kind of rude to want to bail after they helped make Reign three times stronger. Alex really likes her new suit. Lena goes to talk to James and he comes clean on the breaking in but not going into the vault. He also admits that he is Guardian. She admits that her brother didn't have any kryptonite — she figured out how to make it herself. And now Reign knows she needs to kill Ruby to make sure Sam never takes control again.
I'm very conflicted about this episode. There were some good, surprising moments, some nice emotional ones, and then there were some massive convenience moments that just felt like lazy writing. I don't use that term much with the Arrowverse series, so it really surprised me. Covering up a part of the sun shouldn't have an effect on Supergirl. A full eclipse would, but the beginning of one shouldn't have knocked her out — and then for her to just suddenly end up in the same place as Sam and Julia seemed awfully convenient.
The whole mental alternate reality didn't make a lot of sense in the first place, but less so when you make it a place where other exist too… and then they equate it to the mental apartment Kara was in when Brainiac had to help her out. Then there is the fact that they spent a third of the season building up to three Worldkillers and putting together a team around Supergirl only to have two of them killed with no damage to Team Supergirl — with six episodes left in the season. I know the show has gone through some difficulties this season, but this feels like a total concept shift for the season. Oh, and I guess we don't need the Legion hanging around anymore. Their work is done.
The good of the episode was James reaction to Lena and him lying about there not being anymore kryptonite only to find out Lena can make more. I'm not sure how you make a component from another planet… but okay. The Mon-El / J'onn moment was nice. Martians are huggers. And if you are going to wipe out two-thirds of your villains, then making the last one three-times more powerful is a good idea. Then putting her on a path to killer her own daughter is cold… good drama… but cold.
They've now set up the question of "Will Mon-El stay or go?" And the odds that Lena hasn't figured out that Kara is Supergirl have to be slim. Kara's ex-boyfriend is a superhero, her sister works for the DEO… Lena isn't stupid. Check out the trailer for the next episode below.