- For the eponymous character, see Eliza Harmon.
- For the eponymous character, see Trajectory (split personality).
- "A lady speedster? It only took us two years, but we finally got one, a lady speedster. Hey, here's a question for you, purely scientific, was she good-looking?"
- —Cisco Ramon to Barry Allen on the "lady speedster"
Deciding to blow off some steam, Barry and the team head out for a night on the town, only to encounter an unexpected speedster who is up to no good. Iris is challenged by an assignment from her new boss, and is surprised when friction turns to flirtation.
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow
- Carlos Valdes as Cisco Ramon/Vibe
- Keiynan Lonsdale as Wally West
- Tom Cavanagh as Dr. Harry Wells
- Jesse L. Martin as Detective Joe West
- Teddy Sears as Jay Garrick/Zoom
- Violett Beane as Jesse Wells
- Tone Bell as Scott Evans
- Allison Paige as Eliza Harmon/Trajectory
Barry is desperately training to get faster, going so far as to jumping canyons, which he fails. Back at S.T.A.R Labs, Cisco Says that Team Flash requires a break and suggests the Club, while Wells does not like the idea of Jesse going, he eventually agrees, although he has her wear his watch as a means to track her. Jesse, Cisco, Caitlin and Barry meet Iris and Wally at the club. Jesse goes to the bathroom and attempts to turn off the clock. She tampers it and activates Wells' log and is disturbed. While Cisco and Caitlin dance, Barry and Iris reminisce on the fact that they are married on Earth-2 and in the future. Moments later an unknown Speedster with Flashes lightning robs everyone in the club. Barry chases after her but she escapes. At Central City Picture News, Scott Evens tells everyone that The Flash is now a criminal, while Iris defends him.
At S.T.A.R Labs Jesse goes to her father and plays the log. Wells confirms all the words there are true and Jesse Storms out. Team Flash try to understand how the Female Speedster got her powers, Caitlin and Wells tell Barry and the rest of the team about Velocity 9. Barry grows mad at the team for not telling him about a formula that could make him faster, while Caitlin tells Barry that the formula could end up killing him. The team get word on ‘Bad Flash’ and Barry chases after her, in a fight he quickly loses. Caitlin remembers that when designing Velocity 9, she got help from a fellow scientist, Eliza Harmon. Joe and Caitlin investigate the lab though Eliza denies any of the formula in the lab. Later Eliza starts hearing the voice of Trajectory in her head and takes her final Velocity 9 formula. At S.T.A.R Labs Barry steals the final velocity 9 formula, thinking of taking it, though Wells talks him out of it.
Soon Trajectory infiltrates S.T.A.R Labs, puts Flash in the pipeline, knocks Joe out than takes his gun and holds Wells at gunpoint and requesting more Velocity 9. After holding Jesse at gunpoint, Wells makes more of the Velocity 9 and after making sure it's real, Trajectory leaves. She injects Jesse with V-9 and Wells gives her a blood transfusion to save her. Jesse has had enough of her father's protectiveness, refusing to be protected if other people gets hurt in the process. After a while, the team find out she is attempting to make a bridge collapse, which it does after Flash gets all the civilians off. Barry finds out he has to go Mach 3.3 to jump the bridge, which he achieves and beats Trajectory. He gives her the chance to beat her addiction and prove stronger than Velocity 9, but she refuses. She takes another formula, speeds away and her lightning turns blue, much like Zoom's, she runs away and disintegrates .
Back at S.T.A.R Labs, Wells discovers his watch and that Jesse had left a message, saying she has left on her own to explore Earth-1. She is seen boarding a bus to Opal City. The team discusses about how Trajectory's lightning turned blue and how her body disintegrated shortly afterward as she was running. Barry suggest that the V9 is what causes a speedster's lightning to be blue like Zoom's, which could explain why Zoom is much faster than him. Cisco asks if it means that Zoom is ailing, but Wells realizes the true reason: Zoom is dying. He needs a cure, which is why he wants Barry's speed. Barry assumes that this is the same as Jay. The team starts having suspicions that Jay is Zoom and after Cisco vibes his helmet, they come to the conclusion that the theory is true. Barry, devastated by the revelation, speeds off back to the canyon where he previously trained and screams towards the sky.
- Harry claims that "nothing in life is promised except death". When asked if he is quoting Edgar Allen Poe, he replies with Kanye West. The former might be a slight reference to Harry's actor Tom Cavanagh's role in the Fox series The Following.
- After watching a documentary on the singer Beyoncé, Jesse Wells remarks that they must mean "Senator Knowles", from her Earth.
- This episode marks the first appearance of a female speedster on the series.
- At the end of this episode, Barry starts realizing that one of his mentors ("Jay Garrick") double-crossed him and the rest of Team Flash once again, secretly acting as a villainous speedster. At the very same point in season 1's "" "Dr. Wells" a.k.a. Eobard Thawne/Reverse Flash, murdered Mason Bridge in order to cover his tracks which also didn't go unnoticed by his friend and apprentice, Barry, as well as his team.
- When Joe West first introduces himself to Eliza Harmon as a detective, she responds, "How very Law & Order of you." Jesse L. Martin, the actor who plays Joe, starred as Detective Ed Green on the TV series Law & Order from 1999 to 2008. Danielle Panabaker also guest starred in episodes of Law & Order: Special Victims Unit and Law & Order: Los Angeles.
- Cisco is seen dancing in this episode. In the pre-New 52 comics, Vibe is known as a break-dancer.
- The concept of a rival speedster destroying a bridge through friction was demonstrated in the comics when Bart Allen- Barry's grandson and the active Flash- was confronted by rival new 'hero' the Griffin, who sought to destroy the bridge with the goal of saving those on it to affirm his status as a 'hero', only for Bart to stop the bridge from falling in the first place by repairing the damage at superspeed.
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