The conjoining was caused due to a mix between previously theoretical particles carrying Ronnie Raymond interacting with Professor Martin Stein, who held a scientific object, relating to his transmutation project, F.I.R.E.S.T.O.R.M., the namesake of the pairing. The quantum splicing that keeps Ronnie Raymond and Martin Stein together, and allows them to produce nuclear energy to use their meta-human powers, is referred to the Firestorm Matrix.
On December 11, 2013, Ronnie Raymond was caught in the blast caused by the S.T.A.R. Labs particle accelerator. He was transported along with many previously theoretical particles outside, meeting with Professor Martin Stein, who at the time held a mysterious object relating to his F.I.R.E.S.T.O.R.M. transmutation project. They were then merged in to one person, but due to their unwillingness, Stein's mind took control of Ronnie's body, while Ronnie's mind was suppressed, causing Stein to feel identifiably schizophrenic. With Stein in control, he tried to return to his former home to find his wife, Clarissa. However, due to his appearance as Ronnie Raymond, she didn't recognize him and attempted to call the police, before fleeing.
For the following 10 months, Stein chose to live under a bridge and scavenge through garbage to survive.He frequently began making use of his pyrokinetic powers, described as onlookers as a "burning man". One night, he was accosted by two muggers, who taunted him for appearing cold. He got up and displayed his powers, assuring that he wasn't cold.
A few days before Christmas, Firestorm, likely under the direction of Ronnie, followed Ronnie's fiancée, Caitlin Snow, to the Central City Mall garage. He approached her as she was getting into her car, but ran away when she noticed him. Caitlin caught up to Firestorm, but, unwilling to face her, he covered his hands and head in flames to scare her off and escaped the garage. Later, Caitlin found him with the help of Cisco Ramon by following the ionized particles Firestorm emitted. When Caitlin tried to talk to him, he denied that he was Ronnie Raymond and warned her to get away. Caitlin insisted the need to take him to S.T.A.R. Labs to remind him of his identity, causing him to take her face in his hands and whisper "Firestorm". He distracted the two of them with flames and promptly escaped. That night, Firestorm arrived outside S.T.A.R. Labs, right as The Flash was fighting the Reverse-Flash. He saved The Flash, shooting flames at the Reverse-Flash. When the Reverse-Flash escaped, Ronnie warned Caitlin not to look for him, before flying off by propelling himself with flames.
Some months following, Stein approached Quentin Quale, an old friend of his. He claimed to be looking for help, but Quale hardly recognized him due to his new appearance, and he accidentally lost control of his powers, burning the physicist. Deciding to stake out Stein's wife's house, Dr. Wells and Caitlin waited for Firestorm to appear again. When he did, he was confronted by The Flash, whom he easily defeated by flying into the air and forcing him to drop. When Firestorm was about to strike down Barry, he saw Caitlin and fled. However, Barry had planted a tracking device on Firestorm and was able to find him again. With Clarissa's help, the team convinced Firestorm to go with them for treatment. Wells and Caitlin stabilized Firestorm's mind with a drug cocktail used for dissociative personality disorders. At this point, Stein's personality was dominant, with Ronnie's dormant, although Stein did have knowledge of Ronnie's memories and some of his feelings, including his love for Caitlin. When Wells discovered that Firestorm's body was destabilizing and would soon disintegrate in a massive nuclear explosion, Firestorm decided to isolate himself at a location far away from Central City to avoid harming anyone. Barry and Caitlin arrived with a quantum splicer, which they hoped would harmlessly separate Ronnie and Stein while preventing the explosion. Caitlin activated the splicer, but a huge blast of energy was released from Firestorm's body, engulfing the area.
Fortunately, though creating a giant explosion, Ronnie and Dr. Stein were separated, causing no nuclear residue. For a time, Professor Stein and Ronnie attempted to go their separate ways. However, they soon found themselves being connected and drawn back together. The S.T.A.R. Labs team was able to track down a kidnapped Stein, and in order to defeat General Eiling, Ronnie and Professor Stein decided to make peace with each other and willingly merge back together, giving Ronnie control but allowing Stein to have a voice within. Before Firestorm could knock Eiling down, however, he released an ion grenade, causing the Firestorm Matrix to destabilize momentarily and knock Ronnie down. As Eiling aimed a gun at Firestorm, The Flash quickly intercepted and knocked him to the ground. Initially about to hurt General Eiling, Barry suggested that they head home together, which Ronnie accepted. Back at S.T.A.R. Labs, Barry suggested that Stein and Ronnie try to separate. Trying hard, they were able to succeed. They headed back to Stein's home, where Ronnie and Stein bid their farewells to Caitlin and Clarissa, respectively, telling of their plan to go to Pittsburgh for more information on their condition. Outside, they merged back in to one, expressed their love and flew off.