- "So, which one of you lounge lizards is in the wrong universe?"
- —Killer Frost questioning the people of Earth-2
It acts as the first half of a two-part episode.
|Barry, Wells and Cisco journey to Earth-2 to rescue Wells' daughter, Jesse, from Zoom. Barry is stunned when he runs into Earth-2 Iris and Joe, but nothing prepares him for meeting Killer Frost and Deathstorm. Meanwhile, back on Earth-1, Jay has to take over the Flash's responsibilities when a meta-human nicknamed Geomancer attacks Central City.|
- Grant Gustin as Barry Allen/The Flash and Dr. Barry Allen
- Candice Patton as Iris West and Detective Iris West-Allen
- Danielle Panabaker as Dr. Caitlin Snow and Caitlin Snow/Killer Frost
- Carlos Valdes as Cisco Ramon/Vibe and Francisco Ramon/Reverb
- Keiynan Lonsdale as Wally West
- Tom Cavanagh as Dr. Harrison "Harry" Wells
- Jesse L. Martin as Detective Joe West and Joseph West
Special guest star
- Teddy Sears as Jay Garrick/The Flash
- Demore Barnes as Dr. Henry Hewitt
- Violett Beane as Jesse Wells
- Michelle Harrison as Barry Allen's mother (voice)
- Michael Rowe as Detective Floyd Lawton
- Patrick Sabongui as Captain David Singh and Singh
- Adam Stafford as Adam Fells/Geomancer
- Nicholas Baric as Thug #2
- Mike Carpenter as Thug #1
- Katie Hayashida as Cell Phone Girl
- Zain Meghji as Reporter
- James Mowat as Thug #3
- Joseph David-Jones as Green Arrow (archive footage)
- John Wesley Shipp as The Flash (archive footage)
- Melissa Benoist as Supergirl (archive footage)
- Johnathon Schaech as Jonah Hex (archive footage)
- Ryan Handley as Zoom
- Tony Todd as Zoom (voice)
- Lucas Wolf as Man in the Iron Mask
Before heading to Earth-2, Barry closes all but one of the breaches so that Zoom has no other way to travel between the parallel Earths. Using the last breach at S.T.A.R. Labs, Barry, Cisco, and Dr. Wells travel to Earth-2, but there is a surge of energy that destabilizes the breach. That caused the trio to be stuck on Earth-2 until Caitlin and Jay are able to fix the speed cannon. Barry decides to impersonate his Earth-2 self to get more information on Zoom. He later meets Iris and Joe of Earth-2 and learns that Iris works for the CCPD, while Joe is a singer at a jazz bar. Barry is attacked by Earth-2's Caitlin and Ronnie Raymond, known there as Killer Frost and Deathstorm, agents of Zoom. In the attack, Joe is killed. Cisco attempts to help the police stop the pair, but is confronted by his own evil doppelgänger, "Reverb". Cisco discovers Reverb has greater power, including control over nervous systems. Barry tries to rescue him but is stopped by Reverb and Deathstorm. Zoom arrives, kills Reverb and Deathstorm for harming the Flash, and then imprisons Barry alongside Jesse. Meanwhile, Jay reveals to Caitlin that he invented Velocity 6 on his Earth and that it is the real reason why he lost his speed and is dying.
- When Barry, Cisco, and Harry travel to Earth-2, several cameos are seen in the dimension stream:
- Connor Hawke, a Green Arrow from Earth-16 who later appeared in the episode "".
- The Flash of Earth-90 from the 1990s television series , portrayed by John Wesley Shipp. The emblem of this Flash's suit is backward, much like the ones of Eobard Thawne and Zoom; this was taken as a possible clue by some fans, but it was most likely a production error (a left-right "flop").
- Supergirl from the television series (a reference to Barry's future appearance on the show, in the episode "").
- Grodd, who was deported to Earth-2 by Team Flash in "".
- Jonah Hex, an old west gunslinger. The character would later appear in the DC's Legends of Tomorrow episodes "", "", and "".
- A Legion ring used by the Legion in the 31st century. The shot in question shows Clark Kent's ring kept in the Fortress of Solitude and is later reused in the Supergirl episode "".
- Once arriving on Earth-2, Cisco states, "I've got a feeling we're not in Kansas anymore", referencing the quote from Dorothy Gale in the movie The Wizard of Oz.
- On the telephone at Earth-2 Barry and Iris' house, the names "Eddie", "Bruce", "Hal", and "Diana" appear as speed dial options below "Mom & Dad". The first name may refer to Eddie Thawne, which suggests the character has an Earth-2 doppelgänger. The other 3 names are references to multiple superheroes from the comics: Bruce Wayne who is the alter ego of Batman, Hal Jordan who is the alter-ego of the second Green Lantern, and Diana Prince who is the alter ego of Wonder Woman.
- The mural in the Earth-2 Central City Police Department reads, "A free and just society". This is a reference to the Justice Society of America, a group of superheroes in the comics.
- The Justice Society of America would later be introduced in the DC's Legends of Tomorrow episode "" (and was first name-dropped in "").
- When Cisco wonders what his doppelgänger would be like, he imagines him to be a rich inventor. In the Flashpoint timeline Barry creates later, Cisco himself is a rich inventor and the CEO of Ramon Industries.
- At Jitterbugs, Joseph sings the show tunes "Where or When" and "My Funny Valentine".
- When Reverb tries to convince Cisco to team up with him, Cisco asks him, "Are you Cloud City Vadering me right now?" This is a reference to a famous scene from Star Wars: Episode V – The Empire Strikes Back, when Darth Vader tries to convince Luke Skywalker to join him.
- Moments later, Cisco tells Reverb that he will not go to the Dark Side with him, again referencing the Star Wars franchise.
- Floyd Lawton's failed attack against Reverb is a reference to a scene from the movie Pulp Fiction.
- During a broadcast on Earth-2, the mayor of Central City is referred to as Mayor Snart. This could be a reference to the Earth-1 counterpart of Leonard Snart.
- When Cisco says, "I've seen fire and I've seen rain", he may be quoting lyrics from a song by the band Blood, Sweat & Tears.
- Zoom takes out Reverb the same way Thawne killed Cisco in an alternate timeline in episode "".
- At Jitterbugs, everyone's Meta-human awareness apps go off prior to the arrival of Deathstorm and Killer Frost. It was proven at the CCPD that Barry Allen from Earth-1 can set off the apps as well, yet the apps do not go off when he enters Jitterbugs. The app also fails to go off in Cisco Ramon's presence.
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