The following contains spoilers for The Flash Season 8, Episode 18, "The Man in the Yellow Tie," which aired Wednesday, June 15 on The CW.
Team Flash was surprised by the sudden arrival of a new speedster in Meena Dhawan, one of the leading physicists in the world. Through her own experiments, Meena has gained temporary boosts of super-speed and she sought Barry Allen as a mentor to help hone her abilities towards good. And as Team Flash learns more about Meena’s mysterious power source, a familiar foe resurfaces in Barry’s life with a surprising twist.
Barry coaches Meena while the two tap into their super-speed as they race through a forest, with Meena accidentally striking Barry with some of her artificial lightning when she temporarily loses control of her powers. Back in Central City, Cecile Horton is making a deposit at a bank as it's targeted in an armed robbery. Honing her empathetic powers, Cecile redirects the fear experienced by everyone at the scene to the robbers, completely incapacitating them before they are all apprehended by the authorities.
Concerned about the artificial lightning’s effects on his body, Barry visits Meena at Fast Track Labs and discovers the experimental oscillator giving Meena her speed boost. Barry is shocked to learn that the device was developed by Eobard Thawne, appearing as he did before stealing Harrison Wells’ physical appearance and suffering from amnesia over his supervillain speedster years. Barry visits the imprisoned, powerless Thawne at Lian Yu; he claims to have no idea what Barry is talking about, but identifies the oscillator as a means to tap into the Negative Speed Force. Deducing that the amnesiac Thawne is from an alternate timeline, Barry realizes that the corrosive nature of the Negative Speed Force will corrupt and eventually kill Meena, racing back to Central City to warn her.
After Barry leaves, the imprisoned Thawne is approached by John Diggle, who has become obsessed with opening the mysterious container that crashed near him as he moved away from Star City. Thawne agrees to help the increasingly desperate Diggle open the container in exchange for seeing what’s inside himself, theorizing Diggle was not yet ready for the power stored within. As Diggle connects with the container’s cosmic power and glimpses the possibilities it offers, he rejects it because it would cause him to abandon his family forever -- much to Thawne’s visible frustration.
Intent on making her speed boost permanent, Meena and the amnesiac Thawne modify the oscillator -- but when Thawne is distracted by Barry accusing him of faking his amnesia, Meena uses the device herself. Now corrupted by the Negative Speed Force, Meena sets out to maintain her speed by consuming any power sources that lie in her path. Speaking with Thawne, who reveals the extent of his love for Meena, Barry finally acknowledges that he is not the same man as the evil Reverse-Flash and the two work together to purge the Negative Speed Force from Meena as she attacks a hydroelectric dam.
Meena easily subdues The Flash with her Negative Speed Force but Thawne arrives and reminds her of their love for each other, causing Meena to come back to her senses. As Meena and Thawne set out to develop an oscillator that mitigates the Negative Speed Force’s influence, Team Flash receives a video call from Ray Palmer explaining how this alternate Thawne came into existence. Cecile also takes on her own crimefighting persona to defend Central City, secretly observed by another shadowy figure while Caitlin and Chillblaine continue their illicit procedure to resurrect Killer Frost. And as Team Flash celebrates another victory, the evil Thawne is approached in his cell by Deon Owens, the master of the Still Force, with an offer to fulfill his twisted destiny.
Developed for television by Greg Berlanti, Andrew Kreisberg and Geoff Johns, The Flash airs Wednesdays at 8pm ET/PT on The CW.