“The Nuclear Man” opens with Barry preparing for his date with Linda Park, trying on a number of outfits at super-speed. But for once, Barry arrives to his date on time. The two hit the town by eating Central City’s spiciest tacos. The date’s cut short, however, with a call for the Flash’s aide. Luckily, Barry’s able to save the day in the time it takes Linda to hit the bathroom. Barry’s date continues to face interruptions requiring the Flash, but don’t worry, being the fastest man alive lets him do two things at once. Iris, not noticing, is impressed. She gives Barry a kiss good night.
Elsewhere, Ronnie Raymond—possessed by Dr. Martin Stein—confronts an old colleague. Stein needs his help, but goes nuclear during their interaction, killing the scientist.
The next morning, Barry tells Iris about his slam-dunk date with Linda. While Iris pretends not to mind, she’s clearly upset that Barry’s moved on with another woman. Across town, Joe has Cisco help him investigate the murder of Barry’s mother. They pay a visit to Barry’s old home, where Nora Allen was murdered, to search for any clues.
The rest of the STAR Labs team receives the report on Dr. Stein’s attack against the scientist. As Caitlin reveals that Dr. Stein is in control of Ronnie’s body, Barry recalls that he met the doctor once before while riding the train, on the day that STAR Labs Particle Accelerator exploded. In the flashback of their encounter, Barry and Stein discuss physics and the Particle Accelerator, where Stein is clearly impressed by Barry’s scientific prowess. In the present Harrison Wells, Caitlin, and Barry decide to meet with Stein’s wife to see if she’s heard from him.
As they question Mrs. Stein, Barry shows her a picture of Ronnie. She recognizes him, saying that a young man matching Ronnie’s description broke into her home several months ago, babbling nonsense. But before Mrs. Stein could contact the police, Ronnie/Stein fled the house. She never heard from him after.
Over at Barry’s childhood home, Cisco and Joe continue their investigation. Everything seems normal, except Cisco picks up traces of silver nitrate. He hypothesizes that if Flash and Reverse Flash were in the house, they would have left traces silver nitrate. Cisco collects the microscopic pieces of silver, which when analyzed, could also provide photographic evidence of Flash and Reverse Flash’s appearance. But Cisco isn’t the only one to get lucky. On Barry’s second date with Linda, they skip dinner—and all pretense, really—and take advantage of Barry’s empty home.
Caitlin and Wells, meanwhile, stake out Mrs. Stein’s home in hope that Ronnie/Stein returns. Almost on cue, a bright burst of light explodes from house. The two call Barry, who’s more than a little busy with Linda. Their date is cut short for real this time, as Barry leaves Linda behind to confront Stein. Barry tries reasoning with Stein, but Ronnie’s body erupts in flame and tries killing the Flash. Before he can do so, however, Caitlin arrives and begs him not to do it. Upon seeing Caitlin, the inner part of Ronnie must kick in, as he leaves Barry alive and flees the scene.
Back at Barry’s home, Cisco and Joe analyze the silver nitrate, which projects images from the night of Norah’s murder. In the frozen snapshots, Joe notices that the the living room Norah died in now has new wallpaper. Joe rips the wallpaper back, revealing blood on the walls. Joe knows it must either belong to Flash or Reverse Flash, and collects a sample to analyze.
The next morning, Barry tries apologizing to Linda for ditching on their hook-up. But before he can properly explain, he’s called back into work. Once Barry’s gone, Linda asks Iris about Barry and his dating history. Totally blocking Barry’s game, Iris tell Linda that Barry is still hung up on an unrequited love, and that now may not be the best time to date him.
Barry’s call leads him to Wells and Caitlin, who have found Stein hiding underneath a bridge. They bring in Mrs. Stein, who talks to the professor. The husband and wife connect, calming Stein down and allowing his consciousness to take hold. With a clear head, he says that he can only split himself from Ronnie’s body through nuclear fission. Harrison thinks it’s possible, but isn’t convinced,
Back at STAR Labs, Dr. Stein cleans up from his hobo look and submits himself to the team’s scientific tests. Barry then receives a call from Linda. She tells him that they shouldn’t see each other. Why? Because Iris told Linda that Barry is still hung up on her. Furious, Barry confront Iris about the accusation, claiming that he no longer has romantic feelings for her. Whether he’s bluffing or not is anyone’s guess.
Back at Central City Police Department, Cisco and Joe runs tests on the blood. Joe wants to see if the blood matches Dr. Harrison Wells, insinuating that he thinks Wells is a suspect in Norah’s murder. Joe doesn’t trust Dr. Wells at all, knowing that he keeps secrets. Cisco doesn’t believe Joe’s claims, but Joe says he’s an expert at reading people, and knows Wells is hiding something sinister. Upset, Cisco leaves.
During Caitlin’s tests on Dr. Stein, Dr.Stein tells Caitlin that he can “feel” Ronnie’s memories inside of him. Wells, meanwhile looks at the test’s results and concludes that Ronnie’s body is rejecting Stein’s foreign atoms. When Ronnie flares up, it’s his body’s way of fighting Stein’s foreign particles. Harrison warns that these flares will only get worse, and Ronnie could go nuclear within the next few hours. Wells also says that any attempt to separate the two would result in a nuclear explosion. The only safe way to stop the explosion, Wells explains, is to kill the host body. Translation: Ronnie and Stein have to die in order to save Central City from nuclear holocaust. Caitlin refuses to resort to that. She convinces Wells to let them find an alternative in the few hours they have left.
As Caitlin, Wells, and Cisco work on a solution, Barry and Dr. Stein chat. Stein recalls their previous interaction on the train the year prior. Stein evens manages to offer Barry some not-so-irrelevant dating advice. It convinces Barry to fight for Linda. He goes to Linda’s office, and eats a ghost pepper (which is extremely spicy) to prove his dedication to her. She agrees to give him another chance.
Back at STAR Labs, Wells sneaks into his Reverse Flash chamber and grabs a pistol. He’s prepared to shoot Ronnie and Stein if their experiments do not work. He may not get the chance though, as Stein has fled STAR Labs. With the clock ticking, Barry maybe not have time so save Central City, Stein, or Ronnie.
The team tracks Stein in the Badlands, a no-mans-land that’s miles outside of Central City. Wells then gives Barry a device called quantum splicer, and tells Barry that he must place the splicer on Ronnie’s body to prevent him from going nuclear. Barry takes off for Ronnie, taking Caitlin with him.
As Barry leaves, Cisco’s blood work from the Allen residence comes in. It doesn’t belong to Wells, but an adult Barry Allen.
In the Badlands, Barry and Caitlin find Stein. They try convincing Stein to apply the quantum splicer that will stabilize him. Stein doesn’t want to try it, but Ronnie’s consciousness emerges and kisses Caitlin. Stein allows her to place the splicer on his body, but it doesn’t seems to work. Ronnie’s body begins glowing bright red before it detonates into a nuclear explosion. Barry and Snow manage to outrun the blast, but Ronnie’s detonation doesn’t go undetected by the outside world. A military bunker, led by General Eiling, picks up the blast. The Eilling then says it’s time to call in in project FIRESTORM. Wait, there’s another Firestorm in the Flash universe? Sadly, the General’s comments close out the episode, meaning fans will have to sit on that curveball until later date.