- "Our love story never ends, Barry. Not even with Crisis."
"Iris, it's inevitable."
"Maybe. But whether you vanish or or worse, I am gonna be there with you. I'm gonna be right there with you to the very end, okay? Because the only thing more inevitable than Crisis is us."
- —Barry Allen to Iris West-Allen
It acts as the second half of a two-part episode.
With the Flash freshly infected by the monstrous Bloodwork, Iris and Cisco battle to help Barry take control of himself before he’s lost forever to Ramsey's influence. Meanwhile, the rest of Team Flash fight to reclaim control of Central City from Bloodwork's growing army.
- Grant Gustin as Barry Allen/The Flash/Dark Flash
- Candice Patton as Iris West-Allen
- Danielle Panabaker as Dr. Caitlin Snow/Killer Frost
- Carlos Valdes as Cisco Ramon
- Hartley Sawyer (credit only)
- Danielle Nicolet as Defense Attorney Cecile Horton
- Tom Cavanagh as Dr. Harrison Nash Wells
- Jesse L. Martin as Captain Joe West
- Sendhil Ramamurthy as Dr. Ramsey Rosso/Bloodwork
- Victoria Park as Kamilla Hwang
- Kayla Compton as Allegra Garcia
- Meera Simhan as Rachel Rosso
- Kailey Spear as Terrified Civilian 1
- Samantha Spear as Terrified Civilian 2
- The final scene with Harrison Nash Wells is also used in , and in their final episodes before Crisis. Only in this episode is that scene’s context given.
- Cisco Ramon had Babel Protocols built just in case just in case the Flash ever went "dark" at S.T.A.R. Labs. This is a reference to JLA: Tower of Babel, where Batman similarly made contingency plans for if the Justice League ever went rogue.
- Barry’s infected form is based on his Negative Flash form from the comics, although Cisco refers to him as "Dark Flash". Previously, this nickname was given to Eobard Thawne in his Reichsmen outfit in concept art for Crisis on Earth-X. In the comics, this name refers to Walter West, a doppelgänger of Wally’s that used his speed to become a brutal vigilante.
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