“I Know Who You Are” is episode twenty of season three of The Flash.
This episode opens with Barry confronting Savitar telling him he knows who he is, before the episode goes back 16 hours. Joe and Cecille are arriving at Joe’s after a run; Cecille is training for the marathon and wants to shave another 30 seconds off her time. Joe needs to shave off substantially more. He’s willing to wait for her at the finish line. Cecille says that she loves him and things get a bit awkward until Joe’s phone rings.
At S.T.A.R. Labs, Barry tells Joe, Iris, Cisco, Julian and H.R. about Tracy Brand, who the future version of Barry told him was the theoretical physicist who helped trap Savitar. Dr Brand wins lots of awards, including a Nobel Prize, but didn’t build the trap for another four years. They hope she might already be working on a prototype.
Julian wants to know why they aren’t looking for Caitlin. Future Flash confirmed that Killer Frost teamed up with Savitar, so they need to do everything in their power to stop it happening. So why is Wally swanning off the earth-3 to visit his girlfriend? Julian is told that they are doing everything they can; Cisco says that of Julian wanted her here, he shouldn’t have ripped off her necklace. Tracy Brand has been tracked down; she’s in the CCU theoretical physics programme. H.R. says that they are going to depend on a grad student then. It could be worse.
Cisco, Barry and H.R. arrive at the university and smell something burning. The future Dr Brand has set a bunch of stuff on fire, which Barry extinguishes. Tracy claims she has everything under control. Given that she accidentally lights her blowtorch as she says that, this is not convincing. She was burning all her work, because she failed her dissertation. 10 years down the drain and a pile of debt. Now, Tracy is going to Big Belly Burger; she heard they are hiring.
Tracy’s advisor did not have a high opinion of her work. She is surprised when the others know stuff about it, because she had only shared that with her therapist. The door behind her is currently icing up, then Killer Frost arrives and tries to kill Tracy. Barry gets her out of the way and H.R. hustles Tracy out. Both Barry and Cisco get knocked down, but Barry uses his speed and the blowtorch to drive Killer Frost away. This is considered to be proof that Killer Frost is working with Savitar. And it is. In the previous episode, “The Once and Future Flash”, Savitar met Killer Frost and, when she asked why she should trust him, climbed out of his armour. At that point, having seen Savitar in the flesh, Killer Frost was willing to work with him.
Tracy gives a description of Killer Frost to the artist at CCPD. Barry tells Joe that this makes things more complicated. Tracy is already heading out; she doesn’t think that Killer Frost will come after her again, as it was a random attack. No, it wasn’t.
Savitar is not happy that Killer frost let Tracy get away, after he told her where Tracy would be, and that Barry and Cisco would be there. She might be called Killer Frost, but she hasn’t killed anyone yet. They can both be gods, if Caitlin simply kills Tracy.
Tracy is at Jitters with no money when H.R. approaches and offers to buy her a drink. Tracy says she needs to calm down. With a Zoom. From the secret menu. It doesn’t sound like a drink designed to calm you down. H.R. admits he was at the college earlier, and Cisco is coaching him over an earwig. Julian is with Cisco, and wants to know why Cisco didn’t take the shot he had at Killer Frost. Joe and Barry are waiting in a car. Joe feels like he’s lying to Cecille all the time. Barry tells Joe to tell Cecille the truth then. He will have to at some point, so why not now?
Tracy has been studying speedsters, and has deduced the existence of the Speed Force. Which she believes can be manipulated and controlled. And asks why H.R. is so interested when everyone thinks she’s a crackpot. H.R. definitely doesn’t; in fact, he seems rather struck on Tracy. When H.R.’s coffee freezes solid, he decides it’s time to go. Outside, Killer Frost throws an icicle at Tracy, but Barry intercepts. In the ensuing pursuit, Cisco freezes again when he has a chance to stop Killer Frost, and Barry ends up with an icicle in the leg again. Because Savitar said she couldn’t kill Barry.
At the lab, Tracy is told that Killer Frost is after her because she has teamed up with an evil speedster from the future because Tracy developed the technology to trap the speedster for an eternity. Tracy realises that this means she’s right. She’s also told they need the trap Tracy will build in four years now. Tracy ends up rather rattled.
Cisco has been holding back because he’s scared of killing his best friend. Through the episode, there are more hints about Savitar’s identity, and Barry starts to put things together. A number of things Savitar said in retrospect have different meanings to what it sounded like. Which Savitar knew. Finally, Barry realises who Savitar is, and this can be guessed just before it’s revealed. And it is someone known.