- For other uses of "Killer Frost", see Killer Frost (disambiguation).
- "I know that we love you, and we would do everything that we can to help you. You and I, we've been through too much together to let each other down now. Please, let me help you."
- "Like you helped your mother? Or Wally? Or me? You keep messing with everyone's lives, wrecking everything, and we're left behind to pick up the pieces from your mistakes. Some things you break can't be put back together!"
- —Barry Allen tries to talk down Caitlin Snow form her Killer Frost-induced rampage
|Caitlin uses her powers to save Barry but as her mother predicted, the effort unleashes her inner Killer Frost. Killer Frost goes on a rampage looking for Dr. Alchemy, kidnapping Julian and battling both The Flash and Vibe. Meanwhile, Joe and H.R. have a heart to heart talk.|
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow/Killer Frost
- Carlos Valdes as Cisco Ramon/Vibe
- Keiynan Lonsdale as Wally West
- Tom Cavanagh as H.R. Wells
- Jesse L. Martin as Detective Joe West
Special appearance by
With Barry's life threatened by Savitar, Cisco and Caitlin are convinced by H.R. to go help him with their powers. Cisco takes them to Barry through a breach, giving himself a bad migraine in the process. On Barry's command, Caitlin releases a blast of her ice powers, momentarily making Savitar visible to them as he freezes. However, the evil speedster breaks loose and runs away. Back at S.T.A.R. Labs, Caitlin gives Cisco something for his headache, telling him he will be fine with some rest.
Caitlin later goes to the Central City Police Department, where she finds Joe West in the interrogation room interrogating a follower of Alchemy. Caitlin knocks on the door, interrupting the interrogation. She lies to Joe that Wally has recovered, prompting him to leave. Caitlin then sneaks into the interrogation room, where she then freezes the security camera, causing her hair to partly turn white as a side effect of her powers. Caitlin then asks the follower of Alchemy about Alchemy's whereabouts. Caitlin does not get the answer she seeks and lashes out, catching the attention of two nearby police officers. Caitlin then breaths cold air into the room, creating a mist. She leaves under the cover of her mist and freezes the door handle, locking the policemen inside. Julian Albert witnesses Caitlin leaving the room and tries to talk to her. Caitlin then takes Julian hostage.
Julian is brought to a freezer warehouse, where Caitlin orders him to work on a search for Dr. Alchemy; however, as the terms are too common, she changes the search to Savitar.
Upon hearing about a cryokinetic meta-human kidnapping Julian, Barry and Cisco realize Caitlin has lost control of her powers. They find her using a scanner normally used for the Cold Gun. As the Flash, Barry knocks out Julian and learns Caitlin wants to find Alchemy to get rid of her powers, blaming Barry for causing her to turn into a cold-hearted killer. The police arrive, forcing Barry to get Caitlin out of sight; she severs one of his leg muscles, telling him not to follow.
A few hours later, Barry heals, deciding to pay Julian a visit to keep Caitlin from getting arrested. He promises not to tell Singh, so long as Barry resigns; Julian also tells him about the Savitar search Caitlin had him do.
Barry informs the rest of the team about Caitlin's plan, deciding to have himself and Cisco watch one of the Acolytes, while H.R. and Joe watch the second. H.R. tells Joe during their stakeout that he should trust his gut when it comes to Wally. Caitlin attacks the Acolyte Barry and Cisco were watching, learning that he was told by Savitar that she is destined to become the insane Killer Frost. Barry and Cisco fight Caitlin, who almost kills Barry with a freezing kiss. Cisco knocks her out, allowing Barry to thaw himself out.
Taking Caitlin to the Pipeline, Barry and Cisco learn Joe made the dumb move of getting Wally out of the cocoon too soon, leaving his brain unstable. Needing the old Caitlin back, Barry forces her to try killing him to escape; however, her true compassionate self-reasserts itself, allowing Caitlin to rejoin the team. She devises a serum to heal Wally's brain, bringing him back to his senses.
Back at the hospital, Julian hears Savitar's voice, leading him back to the C.C.P.D. where he pulls out the mask of Dr. Alchemy.
- The episode title was inadvertently revealed by director Kevin Smith himself when he posted an image of the script to his Twitter account. Although he'd covered the title on the front page with a pen, the paper was thin enough to clearly see the episode title printed on the second page of the script.
- The fight between Caitlin and Cisco is similar to what he vibed when she asked him to see her future. There is only one minor difference; in his vibe, she was completely Killer Frost, and he was wearing the full Vibe suit.
- Greg Grunberg and Kevin Smith both serve as co-hosts for the AMC late night talk show comedy Geeking Out.
- After thoroughly beating Barry, Savitar asks him if he has had enough, to which Barry replies, "not even close." This is the same line used in the 12th episode of Season 2, "", where Harry asked if Zoom's had enough after injecting himself with some of Barry's Speed Force energy, and he replies, "not even close."
- When Joe is told to go and help H.R. on the stakeout, the latter describes it as an "all-star team up." which is also the name of the 18th episode of Season 1, "".
- The confrontation between Savitar and Barry bears a striking conceptual resemblance to that between Zoom and Barry in ""; in both cases, a superb speedster reveals a yet unrivaled level of power and is eventually subdued for a short moment before escaping the scene.
- Cisco states he was worried Caitlin would go all "mother of dragons" on them because of her white hair, referencing Daenerys Targaryen from the TV series Game of Thrones.
- In one scene, Killer Frost holds out her finger towards an Alchemy follower and extends it with ice, clearly a homage to a scene from Terminator 2 where T-1000 extends his finger into a metal spike towards a victim's eye.
- The warehouse where Caitlin takes Julian has a truck out front with the company name Ledded Goh Frozen Foods. This is a reference to the 2013 film Frozen in which the main character, Elsa, has the same powers as Caitlin and sings a song called "Let It Go".
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