- "There's nothing you can do about it. This is his origin story, and it's going to happen no matter what you do."
- —Harry Wells to Barry Allen on Eobard Thawne.
When Cisco gets a vibe of Eobard Thawne, Barry and the team don't believe it. But, after an attack at Mercury Labs, Christina McGee confirms that the Reverse Flash is back. Meanwhile, Iris and Francine share a nice moment that brings Iris closer to her brother, Wally.
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow
- Carlos Valdes as Cisco Ramon
- Keiynan Lonsdale as Wally West
- Tom Cavanagh as Dr. Harry Wells
- Jesse L. Martin as Detective Joe West
- Teddy Sears as Jay Garrick and Hunter Zolomon
- Aaron Douglas as Russell Glosson
- Amanda Pays as Tina McGee
- Shantel VanSanten as Patty Spivot
- Vanessa Williams as Francine West
- Matt Letscher as Eobard Thawne/Reverse-Flash
The team discovers The Turtle is dead and Jay suspects Dr. Wells is responsible. Meanwhile, Cisco asks Dr. Wells for help in controlling his vibing abilities. Harrison determines they are connected to Cisco's fear response and triggers a vibe which alerts the team that Eobard Thawne, AKA the Reverse-Flash, is back.
Eobard confronted Dr. McGee at Mercury Labs before the Flash shows up and addresses him by his real name, catching him by surprise. When Barry exclaims that he saw him die, Eobard refuted that he knew which time period the Flash is from and vows to kill him before knocking Barry down and abducting McGee.
Back at S.T.A.R. Labs, Wells surmises that this version of Eobard is from a point in the future before he went back in time to kill Barry's mother and was thus protected from being erased with Eddie's sacrifice; in essence a timeline remnant protected by the Speed Force. Retrofitting goggles to induce Cisco's vibes, the team learns that Eobard is planning to use tachyons to return to his future. Barry arrives and destroys Eobard's tachyon machine. After a race around the city, Barry stops Eobard and imprisons him at the particle accelerator. However, his capture causes a rupture in the timeline that threatens to kill Cisco, so Barry is forced to send Eobard back to the future to save his friend. Meanwhile, Patty discovers Barry is the Flash but, when he refuses to acknowledge it, she chooses to leave the city. Caitlin tries to find a cure for Jay's illness.
- This episode closely resembles the plot of the 90's comic book storyline The Return of Barry Allen. That storyline also involved a younger version of Eobard Thawne traveling to the past for the first time, while his future self had fought the Flash Family many times already, Thawne arriving at a time when Barry is dead and his former sidekick Wally West is the current Flash. In The Return of Barry Allen, Eobard is actually a huge fan of Barry who got plastic surgery to look like him, but his trip to the past and the trauma of learning about his 'future' death at Barry's hands scramble his memories to the point where he believes that he is Barry. His true identity is eventually exposed, prompting Wally to trick him into returning to his own time, with his fear and the trauma of the recent events addling Thawne's memories of his time in the past while contributing to his future vendetta against Barry.
- The goggles Wells gives to Cisco resemble the comic book outfit of Vibe in the New 52.
- Tina is supposed to die at 9:52 PM; another reference to the number 52.
- Cisco says "Bye, Felicia" to Eobard, a reference to the 1995 film Friday in which the quote is used.
- Jay Garrick's Earth-1 counterpart Hunter Zolomon is a reference to another comic book character. In the comics, Zolomon is a criminal profiler and a close friend of Wally West. He goes on to become Zoom after an accident involving the Cosmic Treadmill.
- The events of this episode are later referenced in the episode "", in which Barry repeated the process that he and Eobard Thawne used with Supergirl in order to return to his Earth.
- This episode introduces the concept of time remnants into the universe, which becomes one of the important plot points of this season.
- When Cisco goes to visit Eobard Thawne in the pipeline, he tells him that he was the one that stopped him and it's because of Thawne that he has his powers. In the season 1 finale "", Thawne disguised as Wells tells Cisco "I wasn't sure till just now" when he learns that Cisco can see through the vibrations of the universe. The events of Cisco telling past Thawne in the cell about his powers could be the reason behind Thawne/Wells saying this.
- Cisco has two nosebleeds prior to his seizure. Each time, the blood doesn't look like it came from his nose but is painted on when the camera shows Cisco.
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