- "For every person you save, there's going to be someone you can't."
- —Joe tells Barry the consequences of being a hero
As Barry and the team at S.T.A.R. Labs work to capture Kyle Nimbus, a.k.a. The Mist, a dangerous new meta-human with the power to become a toxic gas, they revisit the painful night the particle accelerator exploded and killed Caitlin's fiancé, Ronnie. Meanwhile, Joe decides to finally visit Henry in jail after all these years, but things take a dangerous turn when Kyle shows up looking to punish Joe for arresting him years ago. Meanwhile, Iris and Eddie continue to hide their relationship from Joe.
Barry and Iris are leaving a movie theater. Iris is talking about how intrigued she is by the Red Streak, at which point Barry tries to change the subject. Iris gets a call from Eddie while Barry fields one from Cisco informing him there is a high speed chase. Iris is turned with her back to Barry and Barry uses the chance to speed off.
A police car is in pursuit of a pickup and the driver is armed with a gun. The Red Streak intervenes and the policemen are left astonished as where to the driver of the pickup has disappeared. The policemen turn to see the driver handcuffed in the back of the police vehicle. Barry runs back to the theater and Iris doesn't even know he was gone.
One of the crime families of Central City is sitting down to a meeting. The door is locked as the crime boss starts the meeting;he is engulfed by a green cloud and suffocates. The rest of the crime family is also attacked by the cloud and die.
Joe and Barry are in the crime lab at the police station. Joe sees all of the work Barry has done on his mother's case and he tells Barry that he is going to reopen the case of Nora's murder. Eddie interrupts, telling them of the homicide in which the crime family died. Barry quickly suspects that it was a meta-human who attacked the men.
Barry and Joe have joined the team at S.T.A.R labs where Dr. Wells, Caitlin, and Cisco speculate how the meta-human is controlling the gas. The trio talks excitedly about the meta-human when Barry tells Joe they get really excited about these sort of things. Joe says he's only excited about putting criminals behind bars and Iron Heights Prison isn't equipped to hold meta-humans. Cisco says there is one place that might hold them. Dr. Wells agrees but Caitlin is upset and says it's cordoned off. When Barry asks them what place they're all talking about, Dr. Wells answers—the particle accelerator.
In a flashback, on the night of the particle accelerator explosion, Cisco informs Dr. Wells inside of S.T.A.R labs that there is a thunderstorm in the area, but Dr. Wells shrugs it off. Caitlin and her fiancé, Ronnie, are discussing where to go to on their honeymoon. Dr. Wells gathers everyone as he turns the particle accelerator on. Cisco says he thought when the particle accelerator is turned on there would be a loud bang, when Ronnie, who is one of the engineers, informs him that they would all be in big trouble if there was a loud bang. Dr. Wells pops a bottle of champagne, and some of the champagne floats out of the bottle then falls to the ground. Suddenly the lights go out and there is a loud bang.
Back in the present, Barry notices Caitlin is upset and tells Dr. Wells he can use her help in identifying the poisonous gas, knowing she doesn't want to go down to the particle accelerator with Cisco and Dr. Wells
At home, Joe reviews the tapes of Henry Allen's testimony where he tells the interrogator that he didn't kill his wife. He calls for Joe to tell them that he is telling the truth and that he would never hurt his wife. As Joe is reviewing the tape, Eddie stops by the house to surprise Iris, not knowing Joe is there. Iris opens the door and indicates that Joe is home. Eddie quickly recovers and gives an explanation as to why he is there. Joe is suspicious of the explanation. Before Eddie leaves, he asks Iris when can they tell Joe they're dating, but Iris says her dad will kill her. Eddie tells Iris that she is killing their relationship.
At S.T.A.R. Labs, Cisco and Dr. Wells visit the particle accelerator to figure out how to turn it into a makeshift prison. As Dr. Wells leaves to go and check on something, Cisco experiences a flashback. In the flashback, he and Ronnie are running down to the particle accelerator to shut it down manually. Caitlin doesn't want Ronnie to go but he says he has to. Ronnie tells Cisco he will go and shut down the accelerator, but if he isn't back after two minutes, Cisco needs to initiate lock down. Ronnie is not fast enough and Cisco is forced to close the door to the accelerator to contain the explosion.
Barry and Caitlin are busy processing the gas at the Central City Police Department. He asks her what was Ronnie like. She tells him that they met when he was building the accelerator and she was working at S.T.A.R. Labs. She says that Barry might have noticed that she is a little bit guarded of herself, but Ronnie always found a way to make her laugh. She says they were two very different people, like Ronnie always used to say, "they were like fire and ice." Ronnie didn't need to be there that night, he went in to support Caitlin. As the computer finishes analyzing the poisonous gas, they are baffled to find DNA of another person inside the victim.
In a shopping center in Central City, a woman gets into an elevator. As the door is closing, Kyle Nimbus enters the elevator. The woman recognizes Kyle and says she thought he was dead. Kyle transforms into a gas and kills the woman. Barry and Caitlin deduce that the meta-human doesn't control the gas like they originally thought–he is the gas. Barry overhears on the radio that there is a toxic gas attack at the shopping center. He suits up, but Caitlin doesn't want him to go, fearing he might die. Barry races to the mall, and finds that the woman is already dead. He sees the gas disappear through a door and follows it. Barry discovers Kyle and they fight. Barry is no match for Kyle and Kyle poisons him. Cisco and Dr. Wells track Barry's vitals via his suit. Suddenly Barry is standing next to them saying he can't breathe. They put Barry on a bed and cure him, but not before they get a fresh sample of the poisonous gas.
Now that the team have a sample, they can start analyzing it to find the culprit. As they are waiting for the results, Cisco renames Kyle "The Mist". Back at the station, Barry tells Joe what Kyle is capable of. Joe informs Barry that the woman who was killed in the elevator was a judge. Barry feels distraught that he couldn't save the judge, but Joe tells him that he can't save everyone, and for every person he saves, there is going to be one person he can't.
At S.T.A.R. Labs, Barry sees Caitlin is still upset and apologizes for scaring her when he was poisoned. Caitlin tells him that she is scared to go down to the accelerator, fearing the pain and bad memories of the night Ronnie died. Barry accompanies her to the accelerator, and when she sees the it, her memories return to the night the accelerator exploded. She is running down to the accelerator to see where Ronnie is. Cisco tells her he is still inside the accelerator and can't get him out because the facility is in lock down. Before Ronnie went in, he gave Cisco a walkie talkie and Ronnie tells them that he adjusted the vents so that when the accelerator explodes, the blast will go up and not out. Cisco gives the radio to Caitlin as he goes to do some adjustments. Ronnie tells Caitlin that the explosion is inevitable and wants to tell her something, but presumably dies in the accelerator. Back in the present, Caitlin says he saved so many lives that day and nobody will know. She says she didn't want him to be a hero, she wanted him to be her husband. As they embrace, Cisco calls them back and tells them he needs to show them something. The analysis of the gas is complete and identified the toxin as hydrogen cyanide. Dr. Wells tells them that there is a sedative mixed within the molecular structure of the gas. Barry deduces that it must have been a death row inmate. Caitlin searches on the computer and finds that Kyle Nimbus was scheduled to be executed the night of the explosion. The explosion allows him to turn into a poisonous gas whenever he wants. They look for a connection between the crime family and the judge. They find that Kyle was a hitman for the crime family and when he was arrested the mob turned on him and testified against him, and the judge is the one who sentenced him to death. Barry remembers that The Mist said there was still one more person who had to die, and Caitlin finds that Joe was the lead detective in the case.
Barry phones Eddie to find out where Joe is. Eddie tells Barry that Joe is at Iron Heights Prison. At the prison, Joe meets with Henry Allen. At first Henry is not happy to see Joe, but Joe tells him he reopened Nora's case and will get him out of prison. Joe then begins to apologize to Henry for not believing him, but Henry reveals to Joe that his belief in Barry was all he cared for. The Mist shows up and poisons Joe. Barry arrives just in time and gives Joe an antidote. Henry is the first person to see the Flash, but does not recognize him, because Barry is moving his head too fast, causing it to blur. The team at S.T.A.R. Labs tell Barry that The Mist can't stay in a gas form for a long period. Barry fights The Mist outside the prison and finally defeats The Mist when Kyle passes out.
At S.T.A.R. Labs, a prison is built for The Mist until they can figure out how to take away his power. In the hospital, Joe woke up looking at an asleep Barry, he woke up right at the moment and they had a brief chat. Iris and Eddie came to tell him they are a couple but before they are able to, Joe says that he knows, because he's a detective. Eddie asked if he was mad which he responds saying they will have a different conversation if he had a gun, which Eddie then leaves to let them chat and states that he had to look into the witness protection program. Joe ask if Iris is serious about this and she said she really likes him, Joe answers he will try badly not to shoot him. Back at the lab, Dr. Wells is in his wheelchair staring at the Flash's suit. Dr. Wells has a flashback to the night of the explosion. Cisco tells Dr. Wells on the intercom that Ronnie did it, Dr. Wells says he is on his way down. On his way, he stops and enters his secret room. He walks to his panel and opens up a surveillance screen, looking at Barry in his lab at the CCPD as the lightning strikes him. Dr. Wells remarks "See you soon, Barry".
Behind the scenes
- The cinema displayed two films on a board, titled Blue Devil II: Hell to Pay and The Rita Farr Story. In the comics, Blue Devil and Elasti-Girl (whose real name is Rita Farr) were both movie stunt performers who ultimately gained superpowers.
- Waid Boulevard is a reference to Mark Waid, a comics writer who is highly regarded for having written a run of The Flash comics.
- Barry stated that he didn't want a museum built in his name. In the comics, the Flash Museum was dedicated to all heroes who had the alias of The Flash, mainly focusing on Barry Allen.
- Captain Singh asks for the "Orloff case". In the comics, Professor Pytor Orloff is a bio-geneticist who created the Red Trinity, a group of genetically-enhanced individuals with super speed.
- Caitlin Snow stated that she and Ronnie were like "fire and ice", a reference to the two's alter egos in the comics: Killer Frost and Firestorm, respectively, which will appear later on the show.
- The area before the core chamber of the particle accelerator is titled "Reactor Area 52", a reference to The New 52.
- To obscure his identity, Barry vibrates his face into a blur. This is how the comic book first Flash used to keep his identity a secret before the second had a mask.
- Dr. Wells mentions Big Belly Burger, a burger chain restaurant shown occasionally in DC Comics, particularly series featuring Superman.
- When writing her blog at the end of the episode, Iris misspells pastime as "passtime".
- Caitlin calls Barry and talks with him, but when Barry's phone homepage is visible, it shows there is no ongoing call in progress.
- Central City Shopping Mall, the mall that is seen from above in this episode, is an actual mall center located in Mexico in the city of Tlalnepantla called Mundo E. The name Mundo E is seen on a sign at the entrance in the picture.
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