- For the titular character, see Firestorm.
- "He's not even Ronnie anymore... he's Martin Stein walking around in Ronnie's body..."
- —Caitlin Snow explains Firestorm
It acts as the first half of a two-part episode.
|After Ronnie attacks a physicist, Barry and the team realize they need to go after Caitlin's fiancé who is now a dangerous meta-human. They deduce the best way to track Ronnie is to find Dr. Martin Stein, who was working on a project called F.I.R.E.S.T.O.R.M.. Barry struggles to balance his duties as The Flash and his relationship with Linda Park, and Joe enlists Cisco's help to reinvestigate the murder of Nora Allen. General Eiling returns to Central City.|
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow
- Rick Cosnett as Detective Eddie Thawne
- Carlos Valdes as Cisco Ramon
- Tom Cavanagh as Dr. Harrison Wells
- Jesse L. Martin as Detective Joe West
Special guest stars
- Robbie Amell as Firestorm
- Bill Dow as Quentin Quale
- Michelle Harrison as Nora Allen
- Isabella Hofmann as Clarissa Stein
- Malese Jow as Linda Park
- Chase Masterson as Sherry
Barry is getting ready for his date by trying on things from his closet. He then heads out and meets up with Linda. She is impressed with the flower he got her. They head for tacos and he can't take the spice right away. She heads to the bathroom and Barry picks up a call and tells Cisco that he can't because he's on a date. He speeds off and back quickly. As they walk around, Linda receives a call and walks away. Barry receives a call from Joe and he has to speed off and return quickly. At the end of the night, he and Linda kiss.
A man named Quentin Quale is approached by the body of Ronnie. He mentions to the man things he would know, allowing him to know that he is Martin Stein. He then says that he needs help.
The next day at Jitters, Barry is happy for his second date that night and talks about it with Cisco and Caitlin when he is approached by Iris. The news brings up burning man prior to Cisco heading to meet Joe. Cisco is brought into the Allen home where they meet Sherry.
At S.T.A.R. Labs, Harrison,Barry, and Caitlin are talking about finding Martin/Ronnie. Upon learning what Stein looks like, Barry remembers how he met Stein on the train upon his return to town before the explosion. During the explosion, Stein is knocked around whilst carrying F.I.R.E.S.T.O.R.M.
Barry, Caitlin, and Harrison head to talk to Clarissa Stein about her husband. When Barry shows her a photograph of Ronnie, she mentions seeing him. A month after the explosion, Martin is in Ronnie's body and showed up at the Stein him. He begins to freak out so she calls the police. In the present, they talk about staking out the home but Barry has a date.
Inside the old Allen home, Joe is being hit on while Cisco is trying to find some evidence. They find some on the vintage mirror that was the same one from back then.
Barry and Linda meet up at Joe's home. He mentions plans but she grabs him by his shirt and kiss him.
Harrison and Caitlin are outside eating fries. Caitlin wonders why Ronnie's body had gone there.
Barry and Linda are making out when he vibrates. They continue making out when his phone went off and he picks it up and notices a text saying Firestorm. Barry then rushes off, confusing Lina. He meets up with Stein, who flames on almost immediately. Caitlin and Harrison follow at a distance. Stein takes off when Caitlin tries to stop him from hurting Barry.
Cisco gets imagines from the mirror that allow them to see what happened that night. Cisco turns it into a hologram. They come across an extra blood splatter so Cisco collects the DNA.
Barry heads over to see Linda the next day. Barry learns that due to him rushing off, he blew it the night before. He tries to fix things with her and after Barry is called away again, so Linda asks Iris about Barry. Iris vaguely mentions how Barry had been in an unrequainted love with someone for a while so he might not be ready for a relationship.
Approaching Stein, Caitlin brings Clarissa to talk to Stein about splitting them. Stein allows them to take him to S.T.A.R. Labs. He is given anti psychotics and is clear headed. He uses an old nickname of Caitlin's before being asked not to use it.
Barry receives a call from Linda who brings up him being hung up on Iris. It results in Barry rushing off to talk to Iris about it. He is upset with her for possibly ruining it. He says that he doesn't have the feelings for her anymore, something that upsets Iris. He then heads out.
Inside Barry's lab, Joe and Cisco are testing the blood types when Joe brings up testing it against Wells. Cisco is against it and rushes off.
Caitlin is taking tests Ronnie/Stein's temp. She asks if she could talk to Ronnie but he said that he could remember things from Ronnie's life. Caitlin is then called away. Harrison tells them that Ronnie's body is rejecting Stein.
Cisco shows up and learns that they had Ronnie/Stein. Harrison mentions that the body is destabilizing. Caitlin says they need to use the last hours to find a cure. Wells rushes off as Barry approaches Stein. They talk not just about the train but about Albert Einstein.
Barry then rushes off and heads to talk to Linda. He shows up with a very hot pepper and tells her he'll eat it if she won't go out with him. He then pukes it up and she agrees to go out with him again. From a distance, Iris watches them and is upset.
In his secret room, Wells grabs a gun. He then grabs something to make a splicer. However they learn from Cisco that the professor is gone. Later, they learn that Stein isolated himself as they built a splicer. Caitlin speeds off with Barry to bring the splicer.
Cisco calls Joe and tells him that the blood was from two different people: an adult Barry and an unidentified man.
Barry and Caitlin make it in time and tell Stein about the splicer. Stein in Ronnie's body kisses Caitlin and then right as the bomb went off, Barry sped Caitlin away.
- The episode's title is a reference to the debut volume of Firestorm, Firestorm - The Nuclear Man.
- Joe mentions to Barry about someone attempting suicide on 52nd and Waid. This is a reference to the New 52 storyline of the DC Comics.
- When he encounters Firestorm outside the Steins' house, Barry asks him not to "flame on." This phrase is utilized by Johnny Storm/Human Torch, a Marvel Comics character of the Fantastic Four with similar powers to Firestorm, when activating his powers.
- When Barry mentions his plan for their second date to Linda, he mentions "Mal Duncan, who's playing at an underground jazz club". In the DC comics, Malcolm "Mal" Duncan was one of DC's first black superheroes. One of his aliases was Hornblower, as he could play a horn which opened up multi-dimensional portals. During his time spent with the Teen Titans, Mal owned a nightclub which was frequented by his teammates.
- Harrison Wells mentions the "Firestorm Matrix" inside Ronnie and Professor Stein. In the DC comics, the Firestorm Matrix is an established plot point with in-depth details as to how the Firestorm persona works, similar to the Speed Force with speedsters.
- The Quantum splicer, which is shaped like the Firestorm symbol, can also be seen on the front of the door of the S.T.A.R. Labs make-shift prison.
- Cisco wears a T-shirt that references The Big Bang Theory, perhaps as a homage to Sheldon Cooper, who regularly references The Flash.
- Barry claims the Naga Jolokia pepper is "the world's hottest pepper". At the time of episode production and release, the world's hottest pepper was the Carolina Reaper, approximately 35% hotter than the Naga Jolokia.
- While Joe is on the phone with Cisco, "Answer" and "Ignore" can be seen on his screen, as if Joe had never picked up the phone in the first place.
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