“Reign” begins with Samantha waking up from a nightmare. Ruby rushes in to comfort her mother, and we find out that Samantha has no memory of going into the desert to discover more about her alter ego Reign.
At the DEO, the group is talking about the 31st century with Mon-El and Imra. The latter informs that her group — the Legion of Super-Heroes — were founded to carry on Supergirl’s mission. Mon-El explains they mistakenly traveled back in time, and he wants the DEO’s help of repairing their ship so they can go back into the future.
Kara invites Mon-El and Imra over to her apartment for a holiday party, but the couple declines the invite.
Most of the DEO crew is there including J’onn, Alex, and Winn, as is Lena Luthor and James Olsen as well as J’onn’s dad M’yrnn. There’s a knock at the door and it’s Samantha and Ruby; they’re running late.
Samantha complains of a rough day at work while Alex pulls Ruby off to the side to answer questions about working with Supergirl.
Samantha starts drinking scotch with Lena and teases her boss about having a fling with James. Kara agrees that she notices the chemistry between James and Lana as well. J’onn approaches Kara and tells her he’s gotten a call, and they have to go.
The two go to a burned field, and Kara flies up to get a bird’s-eye view. She notices a logo burned into the fields — it’s the logo of Reign.
Kara approaches the hologram of her mother about the emblem she saw burnt into the ground. She learns that the emblem is Kryptonian, but finds out there isn’t an exact translation of the logo.
At CatCo, James is watching TV sets where the news is displaying reports that the city has been vandalized with various iterations of the Reign emblem. Lena walks in and tells James that he thinks that Morgan Edge is behind the vandalism.
Lena and James approach Morgan at his office about the vandalism, but Edge claims innocence.
Mon-El and Imra are out having Christmas Eve dinner, and Kara and Winn show up. Noticing that Kara wants to speak to Mon-El alone, Winn pulls Imra aside for a beer. While she’s talking to Mon-El, Kara gets a collect call from Albatross Bay Correctional.
Kara shows up and we find out that it was Thomas Coville — the pastor the lead the Cult of Rao — who called her. He’s muttering various Kryptonian-like sayings before looking up and telling Kara she has a lot to learn.
Coville explains that the end is near, and that it’s Kara’s purpose to fight the devil. Kara explains that the devil isn’t real, but she seems hesitant in her own beliefs.
Samantha’s working at CatCo, and Ruby is hungry and wants to go eat. Samantha explains that she’s too busy to go out so the family nanny will be picking Ruby up. An upset Ruby is eventually calmed down after Samantha talks to her about a few of the memories the two share. Samantha then gives Ruby a present; a necklace of the Supergirl emblem.
Kara’s talking to the team at the DEO about Coville’s call, and Imra walks in and says she can help. Imra pulls Kara aside to talk to her about Kara’s past with Mon-El. Imra reassures her that Mon-El didn’t move on from Kara lightly.
Lena and James are investigating the emblems, still under the impression that it’s something that Morgan Edge has been doing. They’re chased and shot at by a gunman, before James stops him with a ricochet.
We see a drug deal going down by the docks when the lights start going out. The drug dealers are taken out by one by one by an unseen figure until the last dealer starts running away. He’s eventually stopped by the shadowy figure, as we see through the figure’s first-person point of view.
Back at her office, Lena and James are trying to do some research on the man who chased them. They reveal that the shooter previously worked for a conference Morgan Edge used to work for.
As Lena and James turn to look at the wall of TVs, where reporters have begun reporting on a dozen gang member deaths at the docks, Samanta looks at a picture of Edge on Lana’s computer before walking out of the office and ripping apart her shirt, revealing the first look at her Reign costume.
At the DEO, Winn reveals that after running tests on the burnt field, the burn was caused by a Kryptonian and not Morgan Edge.
Mon-El approaches Kara, who walks away because she says upset because she no longer sees love in his eyes.
Edge calls Lenato offer his condolences about her attempted assassination when Reign starts wreaking havoc on his Christmas party. He’s able to hide in a safe room in his office long enough for Reign to fly off trying to find him elsewhere.
Kara tells the news to the crew at the DEO before she flies off trying to find the Kryptonian herself, not knowing that it’s Reign, the alter ego of Samantha.
Morgan Edge calls together a press conference, where he tells reporters that he was attacked by someone. He specifically mentions that he can’t be certain that is wasn’t Supergirl who attacked him. Lena and James are watching the report when the two pour themselves a glass of whiskey.
Before too long, the two put their whiskey town and begin kissing.
Mon-El and Imra are training at the DEO and an upset Kara looks on. Winn and Alex walk in to tell Kara that Reign has left another symbol for her.
Reign’s standing on the top of the CatCo building, where she introduces herself to Supergirl. After talking, the two begin fighting.
Kara and Reign’s fight spills over into a building, where the two interrupt a company holiday party. The two continue fighting as “Deck The Halls” plays on in the background.
The two leave the skyscraper and fight on a cargo ship for a bit before the two fly around National City before stopping to fight on a street downtown. Reign’s able to hit Kara over the head with a metal rod, making her bleed.
Seeing a little girl looking on, Kara gets back up to fight. Reign continues beating on Kara, before flying her to the top of a building. Reign drops Kara off the side of the building, who crashes into the sidewalk below.
The DEO agents swarm the scene and take an unconscious Kara back to the DEO, where she begins crashing.
An excited Ruby runs down the steps at her house to wish her mom a Merry Christmas. As she gets downstairs, she sees her mom — who’s still in her suit — looking out the window.