Disclaimer: I do not own any rights to the television program "Gotham". This television program is produced by Primrose Hill Productions. DC Comics and Warner Brothers Television own the rights to this TV show.
This is an origin story of the Joker. It occurs many years after Jerome was sent to Arkham Asylum.
THE EMERGENCE OF THE JOKER
When we last saw Jerome Valeska, the 17 year old that killed his mother was being questioned at the GCPD. We saw him laughing about this crime at the Precinct. After that shocking display to LT Gordon and the Medical Examiner at questioning, the young man was sent to Arkham Asylum.
About 8 years have passed since he arrived at this notorious place. Now, at the age of 25, he planned his escape from this big building that he called home for this long period of time. He was able to create a strong amount of momentum that was needed to leap and jump out from the asylum to another area nearby. Unfortunately, the area that he landed on was a chemical plant that was located next door to Arkham Asylum. As he attempted to achieve his goal of escaping, he fell into a big vat of chemicals that were a fixture of the plant.
After his scary transformation from Jerome Valeska into the Joker, he sought to destroy any mobsters that had anything to do with keeping this chemical plant in operation.
Please note that The Joker has been, and could possibly always be an unstable man that also displayed a penchant for crime. The accident at the chemical plant has also gradually made him even more insane than he was beforehand.
He started to investigate the history of this plant, and was determined to find out who the former and present occupants of this place were and are.
After he conducted this extensive research, he went back to the chemical plant and found the loyal hooligans from the Maroni Family hanging out there since they had been keeping the place in operation as a front for their crime operations. These men all had criminal records that tied them directly to The Maroni Family. But Oswald Cobblepot, AKA The Penguin has never been indicted on any crimes thus far, and had no warrants out for his arrest from the GCPD. Penguin was in charge of these men. This location was used as a place to safely store the cash, and the plans for their future heists. Since he did not have to hide from the law, Penguin also used Fish Mooney's former club and bar as a legitimate place of business to conduct their many illegitimate and illegal activities.
The Joker continued to dig further into his investigation by kidnapping the men that worked out of the Gotham City Chemical Plant. Through his highly inventive investigative techniques (torture, or threat of unspeakable torture), he does find out Penguin's involvement with this place.
Through his observations of Penguin and his men, he sees the fun and excitement that can be had by living a life of crime. So he plans heists and big scores. He went out into Gotham City, seeking to find henchmen and henchwomen that could work with him to achieve the most elaborate crimes imaginable.
After he finds these henchmen and henchwomen, he labors to become the most lethal of all of Batman's enemies.