The CW says of “Flash Back”:
After the shocking revelation that Zoom is Jay Garrick, Barry is more determined than ever to get back to Earth-2 to stop Zoom forever. Desperate to find a way to increase his speed, Barry decides to travel back in time and masquerade as his earlier self in order to get his arch-nemesis, Dr. Harrison Wells/Eobard Thawne, to teach him how to run faster. However, things don't go as planned and Barry is stunned to face familiar foe Hartley Rathaway, as well as old friend Eddie Thawne. Iris makes peace with her past in order to embrace her future.
Longtime talkbacker Shermdawg recaps last week’s “Trajectory”:
In hopes of improving Barry's speed if he could ever found a way back to Earth-2 to fight Zoom, Team Flash had him practice speed jumps over a waterfall chasm....which like any other time he discovers a new power came into play later in the episode when our villain of the week took out a bridge.
That villain being yet another speedster, not Jesse Quick or Wally West who we thought were next in the pipeline. Nope, it was Eliza Harmon, a former colleague of Caitlin who had helped her with a portion of the Velocity 9 serum. Turns out she was able to determine what Snow was ultimately synthesizing and reverse engineered a sample of her own.
It didn't help matters much Eliza was suffering from schizophrenia, and the voices in her head along with her new found abilities led to the birth of "Trajectory" a similarly dressed speedster whose trail of thefts were soon were pinned on Flash. After running out of her drug, Harmon crashed S.T.A.R. Labs imprisoning Barry and knocking out Joe before putting a gun to Jesse's head ordering Caitlin and Wells to make more.
Her addiction eventually led to her death despite numerous warnings from Team Flash that the serum would gradually break down her body on a cellular level....which it did as she eventually took enough to trigger the same blue lightning Zoom emitted and caused her to hauntingly disintegrate during her final run.
A fate Barry managed to avoid as he first learned of Velocity 9 earlier in the episode and was adamant in using to improve his speed even with the risks involved. Something Wells thankfully talked him out of saying it was a short cut he didn't need to make in a speech Barry later relayed to Harmon when he attempted to offer her help. Despite being unsuccessful, Wells seemed proud of Barry for taking his words to heart.
After Eliza's death, Barry started putting the pieces together regarding the blue lightning, Zoom, Velocity 9, Jay, everything....until they all arrived at the frightening possibility that the friend they saw die in front of them was ultimately the man they were fighting. When Cisco gets a vibe off Jay's Mercury helmet that confirms their fears, Barry races back to the waterfall and screams as he had once again been deceived by someone he trusted.
As all of that went down, Jesse found herself at odds with her father's tactics when she learns he had killed in order to ensure her safety. After getting caught up in the Trajectory affair and being injected with a sample of V9 which was later filtered out of her blood, she decides to skip town and explore this new world she found herself stranded in asking Harrison not to look for her.
The Trajectory events also led Iris down a new path as she found a potential love interest in her Editor-in-chief at Picture News after delaying a smear article on the Flash until it was clear there was another meta-human in the mix....and he had once again saved the day.
