Kid Flash returns to Central City with a Zen attitude and new tricks up his sleeve. While thrilled to see his family again, Wally confides to Barry that he returned because he thinks there is something wrong with the Speed Force. Meanwhile, Cisco returns from his fact-finding mission across Earth-Prime.
- Alone with the Psycho:
- Cisco is alone with Thawne possessing Nash's body and only survives because Thawne cannot use his powers yet.
- Kamilla, upon realizing that something is wrong with "Iris", realizes that she is completely alone with her and promptly gets shot by the mirror gun.
- And Your Little Dog, Too!: Thawne threatens Barry and Cisco by telling them that he would kill their family and friends, starting with Nash.
- Ax-Crazy: Thawne comes back more homicidal than ever. When finding Cisco, he immediately attempts to kill him.
- Back for the Dead: The Speed Force as Nora Allen returns and Barry watches her die.
- Big Damn Heroes:
- Wally uses special speed tricks to save people in a helicopter and uses his engineering knowledge to disassemble it to avoid collateral damage.
- Cecile saves Cisco from Thawne by knocking him unconscious.
- Becoming the Mask: The Speed Force, which has mostly operated on Blue-and-Orange Morality, tells Barry that it grew to love him on its death bed like Nora Allen would have.
- Brought Down to Badass: When Thawne is possessing Nash he doesn't have his powers - he is still able to overpower Cisco, however.
- The Bus Came Back: Wally West (Keiynan Lonsdale) appears for the first time since "Nora", and Cisco Ramon (Carlos Valdes) returns after three episodes. Thawne also comes back by possessing Nash.
- But Now I Must Go: Wally leaves again at the end of the episode.
- Card-Carrying Villain:
- Chekhov's Gun: Cisco's camera and the photo stored in it are important later on when Kamilla discovers something off about "Iris".
- Continuity Nod:
- To help dispel the Turtle's powers, Cisco creates a serum derived from the Velocity formula.
- Barry uses Thawne's (formerly Tina McGee's) tachyon device in order to briefly get back some of his lost speed and return to the Speed Force.
- Cisco believes all doppelgängers that landed on Earth-Prime to be dead from the same affliction that affected Beth and Alice on Batwoman.
- Wally remembers how the Speed Force appeared before everyone in the form of Barry's mother at the end of Season 3.
- Mirror Iris brings up the Bloodwork incident where Barry attacked the Speed Force's avatar.
- Demonic Possession: Thawne somehow manages to possess Nash.
- Diabolus ex Nihilo: Somewhat with Thawne, who all of a sudden unexpectedly returns as a major threat for the team to deal with while they are already in the midst of their ongoing troubles with Black Hole, Mirror Iris and Eva, and whatever the hell Sue is up to. However, this is a bit downplayed in that Thawne was at least seemingly foreshadowed by the alternate versions of Wells that Nash had been seeing for a few weeks.
- Dynamic Entry: Wally, as Barry notes, when he is stopping the helicopter crash.
- Failed Attempt at Drama: Cisco dramatically names the Villain of the Week as "Turtle 2.0"...and Barry stares at him for a moment before stating that he was waiting for Cisco to announce a better name. The background music even grinds to a halt.
- Foreshadowing: The green lightning during the initial forays into the Speed Force let the viewers know that The Spectre is somehow involved in the current predicament.
- Glamour Failure: A digital photograph reveals Mirror Iris's true nature, explaining her abhorrence to having her picture taken earlier.
- Hoist by His Own Petard:
- Unlike using his appearance-stealing cord
as he's done in the past to impersonate other people, the as-of-yet unexplained method by which Thawne possessed Nash has robbed him of his powers (which he seems surprised at), making him fairly easy for the heroes to subdue and throw in the pipeline. - Thawne creating his own Speed Force inspires the heroes to do the same.
- Unlike using his appearance-stealing cord
- Kick the Dog:
- Thawne mocks Cisco by miming a stab at his own heart.
- Albeit unintentionally, Wally pointing out that Barry is the reason for the Speed Force dying. As Joe notes, Barry would see it as this rather than the What the Hell, Hero? that was intended.
- Jerkass Ball: Wally, when reading Barry the riot act, behaves much more like his petulant, childish, pre-third season self than the kind and serene man he's become since then. It takes Joe reminding him of his Character Development for him to start acting like himself.
- Let Me Tell You a Story: Wally tells Barry a Buddhist story while apologizing to him.
- Madness Mantra: Eva once again falls into this and it seems to affect Mirror-Iris too.
- Murder Is the Best Solution: Thawne seems to have little desire to do much other than murder all of Barry's friends and family.
- My God, What Have I Done?: Barry is remorseful about "doing a Reverse Flash" on the Speed Force and accidentally killing it by taking in the Spectre's energy during Crisis.
- Nice Job Breaking It, Hero: Taking the Spectre's energy ended up poisoning the Speed Force.
- No Good Deed Goes Unpunished: Taking in the Spectre's power during Crisis to unlock the Speed Force's full potential ended up poisoning and killing it.
- Noodle Incident: Among other things, Cisco has apparently been to Atlantis.
- NOT!: Nash uses a Not! and goes angrily in a way that resembles Harry, to which Cisco throws something.
- Not His Sled: With all the inspiration this season has taken from The Flash (2016), comic fans may be inclined to think the reappearance of Turtle would introduce the other Forces as what's killing the Speed Force. They end up having nothing to do with it in the end, instead being the result of Crisis.
- Not Now, Kiddo:
- Early in the episode Nash desperately seeks Cisco's assistance in trying to fix the strange "ghosts" he's been seeing, which Cisco initially dismisses. By the time Cisco eventually gets around to trying to help Nash with the issue, he's too late and Thawne has already possessed him.
- Barry also doesn't take Wally's warning about the Speed Force being in trouble seriously at first.
- "Not So Different" Remark: Done by Wally when Barry explains how the Speed Force died."You were trying to save the universe. Any hero would have made that call. (deep breath) I would have made that call".
- Oh, Crap!: Cisco, upon realizing that Nash is possessed by Thawne. Thawne then has a small moment himself when he realizes that his speed isn't working in Nash's body.
- Powers via Possession: Subverted. Being in Nash's body keeps Thawne from accessing his own powers.
- Rapid Aging: How Turtle 2.0 kills her victims, as shown with poor Maria. She traps them in time bubbles that rapidly speed up time for them. This also applies to inanimate objects, seeing how the helicopter suddenly started rusting and crash.
- "Ray of Hope" Ending: The Speed Force is gone, Mirror Iris shot Kamilla, Thawne is possessing Nash and the team has a limited time before Thawne kills them with his powers. However, Barry still can find a way to restore the Speed Force and to save Nash, while the team still has time to learn the truth about mirror Iris before it is too late.
- Red Herring: Barry didn't kill the Speed Force by attacking it while being possessed by Bloodwork - he did it accidentally while absorbing the powers of the Spectre.
- The Reveal: It turns out the "ghosts" of dead Wellses Nash has been seeing were Real After All in a sense, and the one "Wells" that actually isn't, Eobard Thawne, pulled a Grand Theft Me.
- Revenge: Frida Novikov targets everybody who had a hand in deporting her.
- Right Behind Me:
- Caitlin and Kamilla think Barry was the one speeding to the rescue of the falling helicopter, only for Barry to show up right behind them, oblivious to the incident.
- Kamilla discovers the truth about Mirror-Iris then realizes who's at the door behind her.
- Shout-Out: Barry channels his inner Yoda when Wally tells him about the disturbance in the (Speed) Force.
- Spotting the Thread: Wally tells his father that he thinks something is up with Iris.
- The Stinger: Mirror Iris reveals herself to Kamilla and shoots her with the Mirror Gun.
- Surprisingly Realistic Outcome:
- Even without powers, Thawne (possessing Nash) and Cisco are still seasoned fighters, and get into a real brawl.
- Similarly, while Cisco is (as the fight choreography shows) not a very effective fighter, Eobard can to go toe-to-toe with Oliver. If not for Cecile, he would've just killed Cisco the old-fashioned way.
- Superior Successor: Frida Novikov's powers are much more dangerous than the original Turtle's.
- Timed Mission: As Thawne notes in the end, the death of the Speed Force means that Barry will gradually lose his speed permanently in a few weeks' time; however, since Thawne's power comes from his Negative Speed Force, he isn't affected, leaving Team Flash completely defenseless against him (once he finds a way to restore his own powers, that is, which is seemingly inevitable). They need to create a new Speed Force, and fast.
- Took a Level in Badass: Wally displays various abilities that Barry never has, to the point Barry notes he’s going to need Wally to teach him some of them.
- Took a Level in Jerkass: Thawne doesn't even bother putting up a facade anymore, he is just straight-up planning to murder Barry and everybody he ever knew and loved. He is even willing to kill Cisco.
- Wham Episode: The Speed Force dies and Barry and Wally will be depowered when their residual speed energy runs out, Thawne possesses Nash, and Mirror Iris banishes Kamilla.
- Wham Line: Wally mentions he saw Nora in one of his Speed Force-induced visions of the future, meaning she was brought back into existence after Barry survived Crisis.
- What the Hell, Hero?:
- Wally lashes out at Barry for killing the Speed Force and not coming to him earlier.
- Joe in turn reminds Wally at how he was back when he met him and that he has a responsibility to apologize to Barry, which he does.
- Caitlin calls out Cisco for being totally dismissive of his friends since his return.
- You Did the Right Thing: As Barry bemoans that his use of the Speed Force to bring back the multiverse is what has ultimately killed it, both Wally and the Speed Force itself assure him that he made the right call.
