This week on "The Flash," Caitlin (Danielle Panabaker) went full-on Killer Frost after using her powers to save Barry (Grant Gustin). Towards the end of the episode, The CW show finally revealed Doctor Alchemy's face, and it turns out that it's someone who has been working very closely with Barry.

After using her powers to free Barry from Savitar's (Andre Tricoteux) crushing grip, Caitlin's personality began to change, with her kidnapping Julian (Tom Felton) and threatening to freeze him if he doesn't find more of Alchemy's followers. The Flash was able to save Julian, and later, Barry teamed up with Cisco (Carlos Valdes) to stop Caitlin. But it appeared as though she had several tricks up her sleeve as she gave Barry a cold kiss, which literally made him freeze over.

Eventually, the two were able to stop Caitlin, and at S.T.A.R. Labs, Barry made one last effort to reach the Caitlin that he knows and asked her to kill him. When she realized that she couldn't hurt her friend, Caitlin broke down and became herself again.

Barry visited Julian and implored him not to turn Caitlin in, and the Brit said that he won't, on the condition that Barry must resign from the Central City Police Department. Barry agreed to do so, telling Joe (Jesse L. Martin) and Iris (Candice Patton) that he must do this to protect his friend.

At the hospital, Savitar paid Julian a visit and told him to become his servant once again. Julian then went to his office and opened a locked drawer, revealing Alchemy's mask inside it.

Next week, "The Flash" participates in the four-way DC hero crossover which will see Barry teaming up with Green Arrow (Stephen Amell), Supergirl (Melissa Benoist) and the Legends of Tomorrow crew as they save the world from an alien invasion.

When aliens called the Dominators attack Central City, Barry speeds over to Star City to ask the Green Arrow for help in stopping them. Together, they track down the Legends of Tomorrow, and Barry and Cisco create a breach to bring back a secret weapon — Supergirl. Once all of the heroes are assembled, a shocking secret related to Flashpoint is revealed and suddenly, no one is sure who they can trust.

"The Flash" airs Tuesdays at 8 p.m. ET on The CW.