In this episode of Supergirl, Mon-El is describing the 31st century, where technology is still mostly used for communication, entertainment and killing. When the governments became overwhelmed by chaos and violence, he used Kara as an example and organized The Legion to help keep the peace.
Imra is thrilled to meet Kara after hearing all about her passionate fight for justice and Kara invites them to her holiday party that night. Later, Sam and Kara tease Lena about dragging James under the mistletoe, while she denies feelings for James. Then Kara tells her new besties that she couldn’t have gotten through the year without them.
The party’s interrupted when the DEO is called to investigate a mysterious shape burned into a field on L-Corp land. Kara recognizes it as something she saw at the old Kryptonian ruins, so she consults the hologram of her mother.
By now, the symbol’s appearing all over the city, so even though it’s Christmas Eve, James scrambles to get his reporters to cover it while he mulls over how to “frame the story” so nobody panics. Lena immediately assumes it’s Morgan Edge, so she and James visit his office. He denies involvement and offers them a corporate Christmas gift for their troubles.
Kara meets with Coville, he eagerly explains that she’s facing the end of days. He spent two years in a global pilgrimage leaning about Rao, and thanks to a disgraced Kryptonian priestess at Fort Rozz, he discovered a prophecy predating the Book of Rao that tells of a dark god.
Coville says her coming is foretold in three stages. First is the mark of the beast, which they’re now seeing. Second is the work of the beast, which involves many deaths. Finally comes the reign of the beast, when Worldkiller arrives to bring about the end of times. He tells Kara that it’s her mission to smite this evil, but she blows off his words.
Back at the DEO, Imra offers to put her skills as a 31st century cop to work, and asks Kara if they can talk alone. She tells Kara that at first, all Mon-El talked about was finding his way back to her, and he didn’t look at another woman until years later when he realized he’d never be able to return.
Also having an awkward conversation are Sam and Ruby. Sam says she’ll need to work through Christmas Eve, and when Ruby gets upset, Sam pulls her daughter into her lap and tells her a story about their best Christmas together. Then she gives Ruby her gift: a necklace with a Supergirl pendant. Samantha says she loves her and promises that they’re always stronger together.
In a bad part of town, Lena and James search for evidence that Edge bought Kryptonian technology to make the mystery marks. Then somebody opens fire on them with a laser gun. James has her take cover while he fights off the assailant with his Guardian shield. Meanwhile at the docks, a dozen gang members are murdered by an unseen assailant in a flurry of violence and a swirling black cape.
At CatCo, Sam arrives to make sure Lena’s okay. She says laser gun guy’s connected to Morgan Edge, who likely wanted it to look like Lena had been killed by Kryptonian heat vision. When she hears this, Sam coldly mutters, “He shouldn’t be alive.” As the CatCo people are distracted by news reports of the dockside murders, Sam strides away, revealing the black Reign sigil underneath her shirt.
In the end, Kara decides to be proactive and burns her own symbol into the top of CatCo. Reign answers the call and flies to the rooftop, but before Kara leaves, Alex stops her. Although she’d been trying to get Kara to embrace her humanity, this new threat scares her, and she asks Kara to be a cold, hard alien for this fight.
On top of the CatCo building, Kara sees Reign, who’s legitimately hard to recognize as Samantha with the face-obscuring mask, altered voice. Reign announces that she’s from a time before fathoming, born to cleanse the scourge and deliver the awakening. Too many people on Earth have eluded judgment, so she’s there to dispense it, and she orders Kara to stand down. Kara declines this offer, and they launch into a fight.
Then they fly onto a tanker before landing in the middle of a National City street. Supergirl tries to warn people to stay back, but Reign brains her with a chunk of concrete-studded rebar. Then she tosses a flaming car door at Kara and uses heat vision to blast her onto a nearby rooftop. By now, Kara’s bloody and sporting a black eye. Reign announces that Kara’s no god and she’s no devil. Instead, she’s truth, judgement and death, and with that, she lets Kara fall off the edge of the building. Kara plummets to the ground in slow motion before slamming into the concrete below.
Then we go to Ruby at home, running downstairs and calling “Merry Christmas!” to her mom. But Samantha’s dressed in all black and staring out the window. Her fingers twitch, and when she whips around at the sound of Ruby’s voice it cuts to credits.