SPOILER WARNING: This article contains major spoilers for "Think Fast," the latest episode of The Flash.
The Flash just completely rewrote Killer Frost's history. In "Think Fast," the latest episode, Caitlin Snow asked Cisco to vibe one of her repressed memories, which revealed that her chilly alter ego has existed since her childhood.
The episode kicked off in a familiar setting: Dr. Sharon Finkel's office. During her session, Caitlin attempted to explain her situation without outing herself as Killer Frost. As she spoke to Dr. Finkel, she referred simply to her "lost friend," who recently disappeared from her life. However, Dr. Finkel changed the topic and brought up the idea of repressed memories, suggesting that Caitlin was repressing some traumatic childhood memory. Caitlin laughed her off and explained that this friend had only appeared recently, when she was an adult.
Nevertheless, Caitlin got a flash of a memory as she and Cisco trained with Barry to capitalize on his ability to bring other people into his Speed Force aura. When she got knocked out of super speed, she got a glimpse of a long-forgotten memory, where she fell off her bike and was almost run over by a truck as a man screamed her name. This continued to bother her for the remainder of the episode, up until one of the very last scenes.
As Caitlin and Cisco helped Clifford DeVoe's hostages out of the ARGUS facility, she asked him to vibe this memory. After some initial hesitation, he acquiesced. In the full memory, Caitlin saw the near-accident again, but this time there was a twist: she caught a glimpse of herself in the truck's side-view mirror and saw Killer Frost's face peering back. At the sight of Killer Frost's visage, young Caitlin screamed and the memory ended.
The fact that Killer Frost -- or some version of her -- has existed since Caitlin's childhood has major implications. For one, Killer Frost came to be long before the particle accelerator explosion; this means metahumans may have existed in the Arrowverse before The Flash's game-changing premiere episode. Since the repressed memory seems to do with her father, this could indicate we will learn more about this mysterious figure, who has only been mentioned on the series so far. It wouldn't be surprising, then, if he has something to do with the creation (or perhaps suppression) of Killer Frost. However, we'll probably have to wait until Season 5 to learn more about Killer Frost's true past.
Airing Tuesdays at 8 p.m. ET/PT on The CW, The Flash stars Grant Gustin as Barry Allen, Carlos Valdes as Cisco Ramon, Danielle Panabaker as Caitlin Snow, Candice Patton as Iris West, Jesse L. Martin as Joe West, and Tom Cavanagh as Harry Wells, with guest appearances from Keiynan Lonsdale as Wally West and Hartley Sawyer as the Elongated Man.