Barry continues to train Jesse and when a new meta-human, Mirror Master, appears on the scene he lets her tag along. Mirror Master has teamed up with his old partner, Top, and is looking for Snart to even a score. Jesse is quick to join the chase but defies one of Barry's orders which results in disastrous consequences.
On the night of the particle accelerator explosion, Sam Scudder and Rosa Dillon are in their hideout, when Leonard Snart suddenly arrives, displeased. Scudder and Dillon decide they want out. A fight breaks out, in which Snart defeats the pair and smashes Scudder into a mirror. and is about to shoot Scudder, when the accelerator explodes. Snart and his accomplice leave, while Scudder and Dillon are hit by the dark matter. Dillon raises to find Scudder missing. Three years later, moving men are fixing the place, when one of them sees Scudder in the mirror. As he touches it, Scudder emrges from the miror and demands to know where Snart is.
Team Flash are at S.T.A.R. Labs, examining information about what happened at Scudder's old place. Since Julian Albert tends to work on his own, Joe goes to see him. Jesse suggests training and she and Barry hit the accelerator. Jesse runs about as fast as Barry, but she trips. She recovers, and Barry tells her it's a lesson, since speed doesn't always defeat metas they fight: they need to be in the environment. Jesse then leaves with Wally. Wells intends to return to Earth-2, but Barry, Cisco and Caitlin would like to have him here. Wells suggests for the team to pick up another Harrison Wells from the multiverse.
At C.C.P.D., Joe speaks with Cecile Horton, to see if she has something about alchemy, but she doesn't. Both Joe and Cecil seem to be into each other.
In a bar, Snart's old accomplice is attacked by Scudder. He demands to know where Snart is. The accomplice claims he doesn't know, and Scudder throws him into a mirror and he flies outside from a window. At the same time in joe's house, Barry and Iris are having a romantic evening and they kiss passionately, until Barry hears the door. Joe enters and Barry quickly speeds and remake's the scenery, but Joe already saw it. Iris is baffled, and Barry says he doesn't want to make out with her with Joe seeing them. Iris urges Barry to talk with Joe.
The next day, in C.C.P.D., Snart's accomplice is being interviewed. As he tells his story with Barry and Joe overhearing, they have an ID on the metahuman, who apparently can move inside mirrors, and he wants to revenge on Snart. Only problem is that Snart is long gone (he had joined the Legends and died by then). Barry talks with Joe about him and Iris kissing, but it goes awkward. Joe heads to interview with one last person from Snart's old gang.
At S.T.A.R. Labs, Wells, Cisco and Caitlin are preparing a satellite, which can send a message to multiple earths. They use an equation test difficult enough to find an intelligent Wells. The message is sent through the breach.
Barry tells the team about Scudder, and Wells beats Cisco by naming him Mirror Master (the fact that there already is a Mirror Master on Earth-2). Barry also says he is searching for Snart, but doesn't know his gone. Now he searches Snart's old accomplices, last being Rosa Dillon, now a metahuman capable of generating vertigos, Cisco naming her Top. They get an indication that someone has answered the test in the multiverse. At Iron Heights Joe interviews Dillon alongside Cecile, but she doesn't talk. As Joe and Cecile walk away (and mention they have a lot of IOY's), Scudder suddenly appears inside Dillon's cell, and escapes with Dillon, through the cell window.
The team realize that Scudder can move inside any reflected surface. In their old hideout, Scudder asks Dillon, where Snart is, but Dillon says he is long gone. Scudder also mentions he has lost three years, when he came out from the mirror. He also asks if Dillon has powers. She demonstrates them by inducing vertigo on Scudder, making him dizzy and lose balance as the world spins in his head like a top. Scudder claims, that with Snart gone, they can take over the city, as no one can stop them. But Dillon says there is one who can: the Flash.
Wells, Cisco and Caitlin run a check on various Harrison Wells's, but Wells passes each one. When they see Wells from Earth-19, Cisco and Caitlin think they found him, but Wells passes him too, trusting his gut.
At Joe's house, Barry, Iris, Joe, Jesse and Wally attempt to watch a movie, but Iris stops it and demands to know, what is wrong with Barry and Joe, even though Barry talked with Joe. However, they can't get to the point when they get an alarm, that Central City National Bank is under robbery. Jesse asks to come along, and Barry agrees as long as she follows his lead.
Scudder and Dillon pull off a robbery, when Barry and Jesse arrive. However, they escape through the window, and Barry and Jesse give chase. When they track them inside building windows, Barry tells Jesse to stay on ground, but she follows Barry anyway. The chase ends with Jesse going head-to-head against Dillon on top of a skyscraper, but she uses her powers on Jesse, and she falls from the roof, forcing Barry to catch her. A they get to the ground, Scudder surprises BArry and throws him inside a mirror, trapping him.
Back at the Labs, Jesse blames herself for Barry getting trapped. She leaves and Wally goes to speak with her. Barry attempts to speak, but his words sound like gibberish. Wells realizes that since mirror shows things in reverse, they hear Barry speaking backwards. Cisco attaches a special speaker into the mirror, which translates Barry's speech to them. However, Barry can't get out. Wells and Cisco believe they can get him out, if the manage to freeze the mirror to a cold enough temperature. Jesse is feeling down, when Wally enters. He tells her not to give up, since he thinks Barry made a lot of mistakes too, when he was new to being a speedster. Wally admits that their earlier kiss got interrupte, because he was afraid, but he isn't afraid anymore, and they kiss again, restoring Jesse's confidence. Wells, Cisco and Caitlin attempt to make one of Cisco's cold guns capable to freeze the mirror, but Wells is not in the game, until Cisco and Caitlin mock him, and he finally agrees to call upon the Wells from Earth-19. They then start to build a molecular dispenser that could work with the cold gun. Iris talks with Barry and learns, that Barry was unsure about them and Joe. When Iris asks, what's it like inside the mirror, Barry figures out, how to defeat Scudder. Wells, Cisco and Caitlin arrive and the guys attempt to use the cold gun with the dispenser, but the mirror doesn't freeze enough, and Barry still can't get out. An alarm sounds about a new robbery, and the team rushes to the Cortex, leaving Barry alone with Caitlin. With no one noticing, Caitlin secretly uses her cryogenic powers to freeze the mirror close to absolute zero. The team is surprised, when Barry joins them. Barry and Jesse go for a rematch.
Scudder and Dillon have pulled out a new heist, when they get a phone call, seemingly from Snart. The pair is lured into an old theme park, where Barry and Jesse use mirrors and Snart's hologram to trick the pair. Barry takes on Scudder, while Jesse takes a rematch with Dillon. Dillon uses her powers again, but Jesse, having learned to cope with the vertigo, beats Dillon. Barry manages to trap Scudder into a circle of mirrors, where he can't move normally, since the mirrors create an infinite mirror effect on one another.
Scudder and Dillon are imprisoned in Iron Heights, and Wells' replacement arrives from Earth-19. He is introduced to the team and Wells and Jesse say goodbye to the team, while Wells also tells Cisco, that he molecular dispenser didn't work, leaving Cisco with a mystery to solve. Wells and Jesse leave for Earth-2. Barry arrives home, and says he should get his own place, to which Iris and Joe sound grateful.
Caitlin arrives home and goes to shower. While there, her cryogenic powers are unleashed, and she freezes her shower solid. As she goes to a mirror, she sees her face and hair are starting to transform similar to that of Killer Frost. Horrified, she cuts off the stroke of white hair.
- Barry gives Jesse advice that "when you enter a new environment you gotta case every inch of it, you never run in blind", realizing that he sounds just like Oliver Queen. This is a reference to the first season episode, "", in which Oliver scolds Barry for not doing just this.
- Barry informs Joe that Leonard Snart is gone and was told by Oliver that he left with some "friends of [their's] on a trip", referring to the Legends.
- "Terra Prime", as stated by Earth-17's Harrison Wells, is Latin for "Earth Prime". This a reference to DC's Prime Earth.
- The beginning of the episode is very similar to the beginning of "": both episodes start with a flashback to the night of the Particle Accelerator explosion, in which the villain of the week almost dies but at the last minute gets saved when the Dark Matter hits him and give him powers. That incident also makes the villain get stuck for years in a place where he can't move. Right after that, the present day is shown, in which several people do construction work, which resualts in the villain finally getting free.
- This episode marks the first time Wentworth Miller has appeared as Leonard Snart on since "".
- This episode marks the return of Cecile Horton since her last appearance in the Season 1 episode "".
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