"The Fanatical" begins with Reign threatening Supergirl and the rest of the group from within her holding
At CatCo, Kara confronts James about Lena's recent actions. They're speaking with one of Thomas Coville's followers, Tonya, busts in with Coville's personal journal. She tells James to give it to Supergirl because she thinks Coville's follows are building a bomb. Kara takes the journal from the woman and says she'll get it to Supergirl.
Ruby's now staying at Alex's place. Alex is trying to cheer Ruby up, who's still upset over finding out her mother is Reign. Ruby tells Alex that she wants to be left alone.
Kara and Mon-El suit up and go to a known location of Coville's cult. When they arrive, they stumble upon a pile of ash, which they find out is a human. They notice a shadow on the wall, an effect of a nuke explosion, which proves Tonya's theory of Coville's cult making a nuke.
Gunmen rush CatCo and kidnap Tonya; James follows in pursuit. James arrives at a warehouse and begins fighting with the Coville followers but cops storm the building. They all draw their guns on James and let the Coville people escape.
Tonya and James are eventually let go and the two discuss the racial profiling by the police force.
At the DEO, J'onn notices Alex has been reading parenting books. J'onn offers her some reassurance on dealing with Ruby.
Kara -- as Supergirl -- talks to Lena about the tension between the two. Kara expresses her wish that the two become honest with each other again and Lena reveals she has no interest in becoming friends with Supergirl.
Supergirl flies to CatCo to talk with James and Tonya after their confrontation with Olivia and the other Coville followers. Tanya reveals to Supergirl that she knows that James is Guardian. Supergirl tells Tonya about the pile of ash of Coville's hideout.
Tanya notices the necklace is one of her friends. James gets a notification on his phone: it's Coville's followers. They're threatening to release James' true identity if they don't hand Tanya and the journal back over.
J'onn and Alex take M'yrnn and Ruby to a local arcade. While there, Alex tries to convince Ruby to let her in Ruby's life, but Ruby just wants to go back home. Myrnn starts having an episode, and the group rushes out of the arcade so as not to become a distraction.
Winn is analyzing the Coville journal when he notices some Kryptonian symbols. Kara realizes that the Coville group isn't trying to make a nuclear weapon. Rather, they're trying to make another Worldkiller. Lena realizes that if they can use Coville's information right, they can do some reverse engineering and come up with a cure for Sam.
Lena and James are talking about the police profiling earlier and we find out that James had been racially profiled as young as seven years old. James realizes that he wants to go public with his identity and the two kiss. Tonya interrupts, wanting to speak with James privately.
Tonya tries convincing him not to reveal himself. She thinks he's revealing his identity for her, but James tries to say it's for himself.
Tonya takes the journal and turns herself into the Coville followers. She enters a van where Olivia takes the journal away. Mon-El, posing undercover, also enters the van, playing dumb thinking it's his Lyft driver.
Back at the DEO, Winn reveals that communication with Mon-El and Tonya has been cut off.
The people in the van get to their new hideout, where Olivia and Tonya begin performing a ritual. Mon-El sneaks off and suits up, where he's able to use his powers to sneak out some ingredients of Olivia's concoction so that they can't perform the entire ritual.
Mon-El is able to use a high-pitched frequency to signal Supergirl to his location and she speeds to the location to help Mon-El, but it's too late. Olivia has apparently managed to transform herself into a Worldkiller.
A fight breaks out between Olivia, Kara, Mon-El, and Olivia's followers when James shows up. Kara tries to take away Olivia's totem but is initially unsuccessful. After much talk and persuasion, Kara persuades Olivia to drop the rock but it's too late, the rock has apparently started burning itself into Olivia's skin.
Mon-El is unable to rip the rock off, so Kara is forced to use her laser vision to burn the rock off Olivia's hand.
James apologizes to Kara for putting her in an odd position with Lena earlier. He reveals to Kara that he plans on revealing his identity to the public on his own time.
Back at her apartment, Alex helps Ruby dye her hair blue. Ruby apologizes to Alex for being mad at her earlier and she asks Alex about M'yrnns state. Ruby reveals that she's afraid that whatever happened to her mom is going to happen to her. Alex mentions that she wants to promise that it won't happen to Ruby, but wanting to be as truthful as possible, Alex doesn't make that promise.1comments
Supergirl and Mon-El return to Lena's lair where Lena and Winn explain they've found a meteor in space that contains the necessary material to help cure Sam/Reign. Mon-El offers to take J'onns ship with Kara to go after the meteor in space.
The episode ends with Coville himself returning to the cite of the ritual and expressing his pleasure with how it turned out.