'Supergirl' Recap With Spoilers: "Trinity"

"Trinity" starts with Lena being questioned by the DEO. Kara asks Lena how she was able to keep Sam contained. Lena reveals she used kryptonite in Sam's holding cell.

The Worldkillers have gone to Reign's Fortress of Sanctuary. The trio begins doing a ritual when Sam suddenly wakes up in the pocket dimension we saw last week. She's soon joined by the human alter egos of the other Worldkillers.

Lena calls James and apologizes for keeping secrets from him. She reveals to him that Sam is Reign. She admits that she failed Sam. Kara interrupts their phone call, making Lena promise she has no more kryptonite.

Kara suddenly has a vision of the Worldkillers performing their ritual. She explains to the rest of the DEO that while there isn't an apocalypse in the Book of Rao, the ancient text does explain how the Worldkillers perform a "Blessed Darkness."

They decide that somehow Kara needs to enter the separate pocket dimension to stop the Worldkillers. They trust Brainiac-5 with using his 31st technology to try and broadcast her consciousness into the other realm. It's decided that both Lena and Alex will travel dimensions with Kara.

Sam and Julia — the human alter ego of Purity — are working through the valley when the latter keeps seeing people she's killed. Julia starts breaking down until Sam reminds her to take a deep breath and think of something else to distract her mind. Sam then remembers about her daughter Ruby and proceeds to break down herself.

Kara flies to CatCo to talk to James about Lena storing kryptonite. Kara refuses to completely trust Lena and asks James to snoop around and try finding out if Lena has any more kryptonite.

On the Legion's ship, Alex approaches Lena about the tests she ran on Sam. Lena promises that Ruby is safe. Kara arrives on the ship and Brainiac-5 tells her that he has found a way to broadcast their consciousnesses, but the more people that switch realms, the weaker they get.

Kara pulls Mon-El aside and tells him that if anything starts going south, to pull Alex and Lena out of the dimension. Brainiac-5 starts the process of broadcasting the three into the pocket dimension.

Kara, Lena, and Alex soon happen upon the body of Grace, the alter ego of Pestilence. She's dead.

Brainiac-5 is counting down the time before they have a full solar eclipse, which would mean the Worldkiller's ritual worked, which would plunge the world into darkness.

James suits up as Guardian and with the help of Winn, gets access to Lena's lab.

In the pocket dimension, the group is arguing about Lena's usage of kryptonite when a Kryptonian Demon begins flying around them. Alex is able to use a stick to sare the demon off and the trio continues their search for Sam and Julia.

They soon stumble upon Sam and Julia, who are in some sort of catatonic states, muttering words of nonsense.

James is looking around Lena's lab when he has a change of heart. Not wanting to go behind her back, James tells Winn that nothing's in the lab, although he doesn't search a highly-secured room.

Alex and Lena start talking to Sam, who comes to and realizes who they are. Before too long, Reign finds out that they're talking to Sam and Sam's eyes turn red. Reign's possessed Sam in the pocket dimension.

Sam/Reign begins choking Lena in the alternate realm when we find out Sam's consciousness in now in Reign's body on Earth. She alerts the DEO to the location of the Worldkiller's Fortress. Imra and Brainiac-5 pull the three out of the pocket dimension. They begin to plan an assault on the Fortress.

Winn gifts Alex with a new uniform and some tech.

Kara, Mon-El, Imra, J'onn, and Alex begin fighting the Worldkillers as Lena stays on the Legion ship to help Brainiac-5 and Winn pilot it.

Team Supergirl is getting bested when they're able to convince Purity/Julia to rise up against Reign and Pestilence. Pestilence stabs Purity, but Purity uses her sonic powers on Pestilence, killing her. Purity dies shortly thereafter.

Their spirits are apparently absorbed by Reign before the Fortress of Sanctuary begins crumbling to the ground. Team Supergirl is able to escape.

Back at the DEO, Mon-El and Imra are celebrating about stopping the Worldkillers. They decide it's time to go home. Imra leaves the room and Mon-El appears to have second thoughts.

Kara arrives and tells J'onn and Alex that the Fortress of Sanctuary has disintegrated, but Reign/Sam is nowhere to be found. Winn tells Kara that he and James searched Lena's lab and found no extra traces of kryptonite.

Hearing the fake news, Kara approaches Lena and tells her that their slate is clean.

Lena talks with James at CatCo. James admits that he originally thought Lena was a spitting image of Lex but over the past two years, she's continued to prove him wrong. James reveals to Lena that he's Guardian. He tells her that he broke into her lab on Supergirl's orders. He tells her he didn't go into her vault because he trusts her.

The two hug and Lena reveals the kryptonite wasn't her borther's. Rather, she found out how to make it.

The episode ends with Reign flying above the city saying "Ruby Arias must die."