This is going to be a summary overall. And there's a lot to cover, so bear with me on this.
So it all begins with two Persona Users, both being Wild Card. One wants to protect the world, while the other wants to sink it into insanity and madness. The former being called Isla (also called Yomi) and the latter being Rei. There's also an outside player called the Tormented One, kinda like a throwback to the Anguished One in the Devil Survivor games. He'll be the one that's truly pulling the strings in the plot for his own amusement, but also in service of the eldritch god Azathoth that wishes to spread madness and chaos since it abhors the stagnation the world experiences through modern life.
So Tormented One awakens, Rei, who already had a pretty messed up upbringing. And uses her to hijack a Terminal that can be used to open the world to the Demon World and eventually use it to allow her to reach the Universal Sea of Consciousness to render all of existence mad. Yomi is able to stop her with the help of a Devil Summoner called Raidou Kuzunoha. However, the facility collapses and the demons have broken free to cause chaos. Thus leading the entire world to know that Persona Users and Demons are now a thing, when they were more of a secret.
Unfortunately Rei's use of the Terminal initially has led to the city of the plot's setting to cause all of the inhabitants to lose their minds and start causing chaos and harm to each other. Rei would be horribly wounded after the battle with Yomi and Raidou. Yomi from trying to save the city through the terminal as well, has also been split into three entities. Her body, her soul, and her will. Her soul is sealed in the deepest depths of the Velvet Room. Her body has been hijacked by an angel, who will now take advantage of the chaos to create her own rule to bring forth order. And her will manifested into the form of a girl called Lisa. The body, Isla, took Lisa under her care as an adopted child along side with her own children, Julius and Serenity.
This would soon kickstart the several years where she'd consolidate power and use her powers as a wild card to cleanse the madness and get the population to forget what had happened when they lost their minds. Hence, she puts metaphorical chains over the city. To keep the population docile and divides her power over 8 people to oversee the city's districts. And now she uses the demons as a convenient scapegoat for the city's problems. And eventually the new Persona Users who are dubbed as Psyche-Terrorists. Hence, the current narrative was that the Eternal Divide (the event naming the time when the demons burst into reality), was caused by a Psyche-Terrorist. Hence in defense of the city, she creates the Cadets with Julius as its captain and the Nameless Royal Guard (Kyouji Kuzunoha) as her only Royal Guard in service to her. Persona Users and the like who are awakened by drinking her blood.
During the conflict, many Psyche-Terrorists have been defeated and imprisoned. Some are brainwashed to serve the Government as Cadets. Though most do not need to drink her blood to do so. That's what the Prison Warden is for.
Now that's the prologue so far.
Current plot starts five years later after the start of the Divide.
Cadets open up on the order of Isla relayed by the Nameless Royal Guard. They all awaken at night of the ritual.
Psyche-Terrorist help a reporter try to catch a Psyche-Terrorist known as the Phantom in order to secure an interview with him. But he ends up awakening their powers by throwing demons at them and directing them to a place called the Platinum Playhouse run by a small goat demon called Larsinny.
At the same night this was all going on, two top NPC Cadets, Chris and Claire, are off on a hunt/patrol for demons. Both of them used to be dangerous Psyche-Terrorists known as Helter Skelter and Queen Bee respectively. The former was such a threat that even the Nameless Royal Guard had to step in.
They get ambushed by Rei's worshippers, the Infinite Jest, on the behest of one of the NPC Cadet's cousin, Serah, who wants to bring her back from the brainwashing. But couldn't due to his powerlessness. However, he would also be infected by the madness as he does the worshipper's bidding. At this point the Infinite Jest have been building their power underground and through the net under the noses of the Government. The Infinite Jest attempt to reawaken them. They infect Claire. But could not do the same for Skelter as he was initially powerful to begin with. Though he has awakened and returned to his former self. However, he had to deal with an insane Claire. He was forced to use his own form of "brainwashing" by making her into a Shadow and leaving behind her original corpse for the law enforcement to find. Which ends up with her returning back to the Cadets as if nothing happened, but with him having to escape.