Iris' bachelorette party does not go as planned on The Flash's "Girls Night Out." What begins as a girly dinner with Caitlin, Felicity, and Cecile quickly devolves into a fight for Caitlin's life when her Killer Frost alter-ego rears her ugly head. For a moment, it looks like Killer Frost is taking over the scientist, but by the end of the episode Caitlin is in control of Killer Frost on The Flash. After going full Killer Frost last year and briefly fighting against Team Flash, Caitlin reluctantly returned to S.T.A.R. Labs, having seemingly gained control over her metahuman side. But, as was revealed in "Girl's Night Out," it turns out Killer Frost has been living a secret life, and it might just get her killed.
Before Barry came back from Speed Force prison, Caitlin was working at a bar, but Killer Frost was working for Amunet Black, aka Blacksmith, a metahuman who can manipulate metal into a gauntlet. She left that life behind when she decided to rejoin Team Flash, but just because she's done with Amunet Black doesn't mean Amunet Black is done with her. In "Girls Night Out," Amunet Black calls on Caitlin/Killer Frost, revealing Caitlin's double life to the rest of the team. During her time off from S.T.A.R. Labs, Caitlin struck a deal with Amunet: a drug to help her control her meta abilities in exchange for icy muscle. The drug, it turns out, didn't work, but a deal's a deal, and Amunet expects Caitlin to hold up her end of the bargain. And though Caitlin has successfully been avoiding Amunet thus far, the new baddie's patience has officially run out.
Amunet wants Caitlin's help selling a new drug she acquired thanks to a brand new bus meta. The metahuman, who Amunet is keeping locked up, produces tears that act as a drug, filling users with a feeling of happiness and love. Once Caitlin is faced with Amunet, Killer Frost quickly takes over, but instead of letting Killer Frost do her thing, Iris decides that it was her responsibility to help her friend, and help save a bus meta in the process. And so, Iris' bachelorette party became a night of the girls fighting metahumans — #Feminism.
Appealing to Killer Frost's inner Caitlin, Iris convinces her to work with them to save the new meta. And, unable to reach the boys, who were celebrating Barry's bachelor party with an ill-advised trip to a strip club, Felicity, Iris, Cecile, and Killer Frost are able to successfully save the new meta and temporarily stop Amunet all on their own. And, not only that, but Iris is able to bring Caitlin back from Killer Frost.
By the time the boys are released from jail (it was a crazy night), the girls were safely back at the lab. Killer Frost, who earlier in the episode announces that she's getting ready to skip town to get away from Amunet, has decided to stay in Central City, but she has to do one thing first: tell the rest of the team. Knowing that seeing is believing, Killer Frost transforms into Caitlin right in front of their eyes. The ease with which Killer Frost changes forms suggests that Caitlin might finally be gaining some control over her powers. And not a minute too soon — Team Flash is going to need all the superheroes they can get.
While Iris and Caitlin are busy trying to gain control of Killer Frost, the new bus meta gets away. Or, he would have if the Thinker hadn't appeared out of thin air (or perhaps through a breach between Earths?) just in time to stop him from leaving Central City. He needs the meta to stay where he is for his grand plan to work. Whatever it is, it can't be good.