The world of Blazblue is...complicated, to say the least. Better get a sandwich; this is gonna take a while.
In the year 2100 AD, Humanity would face the first of its greatest challenges with emergence of a creature known as the Black Beast.
The beast had only one goal: complete and total destruction of everything before it. Countries were devastated by its power including Japan and Europe. Humanity was brought to the brink of extinction in its wake. The beast spread a substance known as seithr across the entire planet, which allowed its influence to spread everywhere. Humanity was at a loss for how to deal with the beast, as it was immune to everything thrown at it, nukes, bombs, nothing worked.
Humanity would finally have its chance when the beast went inactive for exactly one year after a man named "Bloodedge" fought it off. During the one-year break, Humanity would recuperate in anticipation of the beast's return, developing many powerful weapons in the process and taking advantage of the abundance of seithr on the planet. One of the Great Sages of the Ishana Wizard school, Konoe A. Mercury aka "Nine", would pass her magical knowledge onto humanity and the magical systems and weapons known as "Ars Magus" were born. She also developed 10 powerful and indestructible weapons known as the Nox Nyctores.
Eventually, the beast resurfaced and humanity fought with its newfound strength and with the aid of six individuals who proved vital in the beast's destruction, Nine being one of them. These individuals would be heralded as the "Six Heroes" and their legend would be told for generations to come. After much struggling, the decade long war came to an end in 2110 AD.
Nearly 100 years after the Dark War, the world was governed by the now established "Novus Orbis Librarium" with hierarchical cities established all over the world as the surface was too ravaged by Seithr to be habitable.
The 5th Hierarchical city, Ibukido, would declare independence from the NOL and establish itself, along with several neighboring cities as the Ikaruga Federation. The NOL, not tolerating this secession, decided to declare war on the Federation and started the first war between Ars Magus users.
At one point, the 9th Hierarchical city of Ikaruga allied themselves with Sector 7, an opposing faction to the NOL, and had a grand plan of making a powerful enough weapon to defeat the NOL, in the Ibukido Cauldron, one of the many Gates in the world that leads to the Boundary, a place beyond time. Before the process could be completed, however, it was destroyed by a certain satellite from space, leaving only one survivor, a young girl without any memories. Said event would be later called "The Burning Fields of Ibukido".
The War went completely one-sided in favor of the NOL, and even when the Ikaruga Federation offered a truce to prevent any more casualties, the NOL declined and continued its assault. Eventually the war would come to an end when the leader of Ikaruga, Tenjo Amanohokosaka, was slain by the man named Jin Kisaragi, one of the heirs to the NOL's royal families. His efforts would earn him the title of "Hero of Ikaruga".
After the war, the NOL imposed a far stricter rule upon the world, and any signs of rebellion and insubordination would be met with death.
NOL facilities are being destroyed one by one, and the cause of it all is one man, known as "Ragna the Bloodedge", aka "The Grim Reaper". Ragna's efforts earn him the class "SS", the highest class of a criminal branded by the NOL and the highest bounty to anyone who brings him in. One of the primary reasons the NOL want to capture Ragna is because he possesses the most powerful grimoire of all, the Azure Grimoire, otherwise known as the BlazBlue and it's what allows Ragna to single handedly face the entire facilities.
Ragna's quest for destruction brings him to the 13th Hierarchical City of Kagutsuchi, his aim being to destroy the facility there as well. It also brings many other characters to the place, most of them going after Ragna for one reason or another. Eventually Ragna would make it to the NOL branch, however he finds that it is strangely deserted. He finds Jin Kisaragi, wielder of the Nox Nyctores Yukianesa, who turns out to be his younger brother. The two engage each other and Ragna manages to win and makes it to the lower level for his true goal, the Cauldron beneath the NOL Branch. Ragna meets Hakumen, one of the Six Heroes who slayed the Black Beast 100 years prior, and was salvaged from the Boundary by Kokonoe, a scientist of Sector 7. Ragna engages him and is initially overwhelmed, but overpowers Hakumen with the power of the Azure Grimoire. Ragna saves a young woman named Noel Vermilion, Jin's secretary and wielder of the Nox Nyctores Bolverk, who was assigned to find him after he went AWOL in his pursuit of Ragna, from being killed. Another young woman appears from the Cauldron named Nu-13 and seems to know Ragna and welcomes him. Exhausted from his previous fights, Ragna is unable to defeat Nu and he is impaled by one of her swords and begins to drag him into the cauldron. Noel manages to save Ragna's life before he falls in.
This action manages to cease a 100 year time loop that had reoccurred thousands of times and lead the world down a new possibility and path. Noel's actions make her the new "Successor of the Azure", aka the "Calamity Trigger". Rachel Alucard, a vampire that followed Ragna's path, uses her "Tsukuyomi Unit" to stop a blast by the satellite/Nox Nyctores "Gigant: Takemikazuchi" from destroying Kagutsuchi and allow the events to play out. Afterward, Hazama, wielder of the Nox Nyctores Ouroboros and the captain of the NOL's Intelligence Department and who had been assigned to accompany Noel on her search for Jin, reveals himself as the mastermind of these events and has Noel unknowingly "Observe" him, which allows the being known as "Yuuki Terumi" to exist. Terumi is a spirit, and is also a member of the Six Heroes.
Terumi was responsible for burning down the church Ragna, Jin, and their younger sister Saya were raised in. Terumi, through Jin, severed Ragna's right arm and kidnapped Saya and left him to die. Rachel intervened and saved Ragna's life and gave him the Azure Grimoire, which now serves as his new right arm. The trauma of the whole experience bleached his hair white and turned his right eye red. He then was trained under the tutelage of Jubei, another of the Six Heroes who's a talking cat samurai.
An enraged Ragna tries to exact his revenge on Terumi, but the latter escapes to continue his plans. Later on, Hazama/Terumi assigns Tsubaki Yayoi, a close friend of Jin and Noel, the task of killing them for treason against the NOL.
After intervening in the current events, Rachel is confronted by the Takamagahara System, a unit with the ability to make small alterations in the space-time continuum and responsible for attempting to fire Takemikazuchi on Kagutsuchi. Rachel is an observer, a being that is meant to overlook events as they take place, but prevented from interfering with them. Because of her actions, Rachel is stripped of her status and slowly her power as well.
Ragna and Noel briefly travel together, Ragna being slightly put off by Noel's resemblance to his sister Saya. Ragna eventually pushes Noel away so that he can focus on how to kill Terumi. Meanwhile, Jin is being treated for wounds he received in his fight with Ragna as well as being put in custody for his mutinous actions against the NOL. Jin manages to escape, cutting ties with the NOL in the process and returns to Kagutsuchi.
Noel meets up with her old friend Makoto Nanaya, another member of the intelligence department as well as a double agent for the organization Sector 7. After reminiscing, Makoto informs Noel that Tsubaki is coming to Kagutsuchi to kill her and Jin as and warns her to stay away from Hazama. Noel decides to find Tsubaki so that they can resolve things peacefully.
Ragna returns to Orient Town where he encounters a young child with three souls in their body; a bratty young girl named Luna, a timid boy named Sena, and Trinity Glassfille, another one of the Six Heroes sealed in her weapon, the Nox Nyctores Muchourin. Trinity informs Ragna not to confront Terumi as he would not be able to defeat him. Ragna doesn't listen and continues towards finding Terumi, and encounters Lambda-11, another Prime Field Device like Nu but under the control of Kokonoe. Ragna learns from Kokonoe that Noel is also a Prime Field Device but with no memory of it and that Terumi's plan is capture Noel and turn her into the "Sword of the God-slayer". Ragna goes off to find either Noel or Terumi before it's too late.
Noel eventually confronts Tsubaki, whom is conflicted over her friendship with Noel and her duty to the NOL; after unsuccessfully trying to talk her out of following her orders, they fight but Noel ultimately wins and learns that it was Hazama who had issued the order to Tsubaki in the first place. An enraged Noel confronts Hazama at the top of the Kagutsuchi NOL branch but is captured and he begins his plan of "smelting" her into her original form to destroy Amaterasu, the Master Unit responsible for maintaining the world.
Noel is revealed to be the one causing the Continuum Shift, a phenomenon where several "possibilities" of the world are brought up by her powers as the Successor of the Azure. Both Takamagahara and Terumi are observing this phenomenon, basing their plans on what happens in each and every possibility.
Jin, still recovering from his injuries, is found by Makoto and the two are taken in by Bang Shishigami, wielder of the Nox Nyctores Rettenjo, a refugee of the Ikaruga Federation and a disciple of Tenjo. Jin learns that he's been under the influence of "Yukianesa" for years now and the weapon drives homicidal fury in him whenever he's around Ragna. After an encounter with Rachel and subsequent "help" from Valkenhayn, her butler and a member of Six Heroes, Jin starts resisting the sword's influence and finds himself unable to draw it as a result. Nonetheless, he goes toward the Kagutsuchi branch and is confronted by Tsubaki. Tsubaki pleads with Jin to stand down, but he refuses. Hazama then reveals that he put Tsubaki under a spell that allows the Imperator, the head of the NOL, to influence her thoughts and forces her to kill Jin. Jin awakens to the "Power of Order", an ability that destroys any phenomenon based on the user's will, and uses it to forcibly draw Yukianesa from it's sheathe. He manages to fend off Tsubaki, but she's transported away by Phantom, another one of Hazama's conspirators.
Rachel decides that in order to prepare for the worst case scenario, she releases Hakumen from Kokonoe's prison and enlists his help in killing Noel should she awaken as her original self. This act uses up even more of Rachel's power. Rachel is eventually subdued by the Imperator.
Ragna confronts Terumi at the top of the Kagutsuchi branch and attempts to rescue Noel. Ragna activates his Azure Grimoire but finds out that Terumi has his own and knows how to shut down Ragna's as he was the one who invented it in the first place. Ragna is completely overwhelmed by Terumi and can only watch as Noel is awakened as Mu-12, and Terumi sends her straight after Amaterasu. As Terumi is about to deal the finishing blow to Ragna, Lambda intervenes and takes the blow. It's revealed that Kokonoe had put part of Nu's soul within Lambda and dies in Ragna's arms. Ragna absorbs Lambda's Idea Engine, an imitation weapon to a Nox Nyctores. This allows Ragna to bypass the influence Terumi has over his Grimoire and overpower him and goes onto rescue Rachel from her prison as well. Rachel tells Ragna the only way to save Noel is to kill her.
Jin and Hakumen attempt to stop Mu, but are unable to. Ragna shows up and confronts her, trying to appeal to her memories as Noel. Using his grimoire, Ragna sacrifices his left arm to restore Mu to her former self. While Takemagahara observed this event, Terumi and Phantom implant a virus into the system, giving control of it to the Imperator. Shortly after, the Imperator makes her appearance known and Ragna and Jin immediately recognize her as their sister Saya. Saya is working with Terumi, Phantom, as well as Relius Clover, the scientist who created the Prime Field devices. Their goal is destroy the Master Unit and bring about "The Day of Reckoning".
Kokonoe creates a new left arm for Ragna and he departs for Ikaruga Federation to find the means of taking down Saya. Jin trains with Jubei in hopes of mastering the "Power of Order" in order to rescue Tsubaki from the Imperator's grasp. Noel reunites with Makoto and they too wish to help Tsubaki and are taken in by Kokonoe on their way to Ikaruga. Meanwhile, Terumi and Relius watch as "the Thirteen" is regenerating inside the Boundary.
One month after the events in Kagutsuchi, Ragna arrives in Ikaruga to find Kushinada's Lynchpin, a weapon capable of eliminating all seithr and plans on using it to kill Saya. Meanwhile, Kokonoe's assistant, a cyborg named Iron Tager, takes Noel and Makoto to Ikaruga to meet up with Kagura Mutsuki, the head of the NOL's Royal Families and leader of Ikaruga. Kagura and Kokonoe are working together behind the scenes to dethrone the Imperator and restore the world. Jin also arrives in Ikaruga with the intention of saving Tsubaki.
Meanwhile, Saya, Relius and Terumi make their own moves and use the Takamagahara system to cause widespread Phenomenon Interventions all throughout Ikaruga to bring them closer to the Master Unit. Tsubaki, still under the Imperator's curse, also heads to Ikaruga to apprehend Ragna and deal with her former friends.
Sector 7, realizing that Kokonoe is working against them, release the notorious war criminal "Azrael" from his prison. Azrael is tasked with killing Kokonoe and Tager as well as retrieving Ragna's Azure Grimoire and heads to Ikaruga.
Ragna searches for Noel, as she is the only way he can use the Lynchpin for his plans, but is led into a trap by Terumi. Terumi forces Ragna to fight a recently revived Nu; with the battle being as dead even as possible, Ragna attempts to activate the Blazblue to end it quickly, but a Phenomenon Intervention prevents him from doing so as Terumi taunts him. Ragna is transported back to a short time before this confrontation, but still retains the injuries from the battle and passes out from his wounds. When he wakes up, he meets a girl named Celica A. Mercury who heals his injuries and seems to be familiar with him. Ragna finds that his right arm and eye no longer work in Celica's presence and quickly escapes her to continue his goal of finding Noel. Ragna eventually finds Noel, but is confronted by Kagura before he can take her anywhere. Kagura easily defeats Ragna and takes him into custody.
Meanwhile, after dealing with Jin, Hakumen finds Celica lost in the city of Kazamotsu; she apparently knows Hakumen well. He decides to take her to Valkenhayn and meets Jubei along the way, but then Rachel appears and teleports them all to her palace, where Valkenhayn and Trinity await their presence. The remaining Six Heroes, with the assistance of Rachel, decide to eliminate Terumi once and for all, and formulate a plan on doing so. Since Terumi dwells inside Hazama, they plan on bringing Terumi out in the open so they can kill him directly using Trinity's Muchourin to materialize a body for him. Eventually they succeed on bringing Terumi out, but he, now having his own body, flees from them.
Kagura's assistant Hibiki Kohaku successfully convinces Jin to assist in his plan of overthrowing the Imperator under the condition of letting him lead the operation to save Tsubaki. Kagura also convinces Ragna to join his cause as long as he can obtain Kushinada's Lynchpin to destroy Saya. Kagura devises a plan for both saving Tsubaki as well as the impending threat of Azrael on the horizon. Kagura, who is revealed to also be a disciple of Tenjo, introduces Bang to Tenjo's son Homura Amanohosaka, the rightful heir to the Imperator's throne. To fulfill his master's legacy, Bang goes to where Kushinada's Lynchpin is and waits for someone who's worthy of using it.
That same night, Rachel assists Noel in controlling her powers as the "God-slayer". Noel accepts her other half and plans on using her abilities to save Tsubaki.
The next day, the operation begins with two specific teams; Ragna, Kagura, Rachel, Celica, and Kokonoe will deal with Azrael while Jin, Makoto, and Noel will deal with Tsubaki. Kagura holds a mock tournament with Ragna's bounty as the prize to lure both Azrael and Tsubaki out, which proves successful. A contestant named Bullet enters the tournament in hopes of finding Kokonoe for answers about the death of her squad and ends up facing Azrael. He reveals that he was the one who slaughtered Bullet's squad and provokes her into attacking and quickly overwhelms her. Ragna quickly intervenes before Azrael can kill her and holds him off while Kokonoe begins her plan to seal him away again. Ragna holds his own, but Celica's presence means he can only fight with one arm and half blind. Azrael attempts to provoke Ragna by attacking Celica, which causes Ragna to become enraged and nearly activate the Blazblue, but he manages to keep it under control. Kagura takes Ragna's place and engages Azrael and discovers that he cannot attack anyone without a will to fight and so he surrenders, preventing Azrael from retaliating and Kokonoe successfully seals him in another dimension.
Shortly after, Tsubaki attempts to apprehend Ragna, but is led into a trap set by Kokonoe, Jin, Makoto, and Noel, who attempt to get Tsubaki to come to her senses. Jin learned from Hakumen that Tsubaki's weapon, the Izayoi, has two different forms and functions. The default form, "Sealed Armament", allows the user to not be observable by any forms of observation, while its true form, "Zero-Type", allows it to kill anybody, even immortals with the ability called the "Immortal Breaker". Jin's plan is to overwhelm Tsubaki and reveal the Izayoi's true form with Noel's power as the Successor of the Azure and undo the Imperator's curse. Makoto first brings Tsubaki to her physical limits and Noel is successfully able to recognize Izayoi's true form and coerce Tsubaki into transforming. Jin is the last to face Tsubaki and the Imperator forces her to activate Immortal Breaker against her will. Tsubaki stabs Jin with her sword and the trauma helps her overcome the Imperator's influence. Because of the Power of Order, Jin is immune to Immortal Breaker's effects and survives the ordeal, successful in bringing Tsubaki back to their side.
After successfully dealing with their two biggest threats, Rachel, with the assistance of Celica and Noel, sends Ragna back to the period of the Dark War and where the Black Beast was still active in hopes of helping him find a means to activating Kushinada's Lynchpin. Ragna learns from Nine, who is revealed to be Kokonoe's mother, that Celica isn't normally from the present timeline, but a time copy created by Kokonoe for activating the Lynchpin as her soul is the catalyst. Ragna refuses to use Celica's soul for the Lynchpin and learns from Rachel that Bang's Rettenjo can act as a substitute as it holds Tenjo's soul. The heroes then begin their plans on eliminating the Imperator.
Due to Celica's nature, she's called a Chronophantasma, or a "Phantom of Time", who's immune to any effects of Phenomenon Intervention. Rachel and presumably Noel are also clumped into the category.
Ragna, Celica, and Noel head to where the Lynchpin is located and encounter Bang. Bang fights Ragna to see if he is worthy of using his master's legacy and Ragna proves as such. Afterward, they are attacked by Relius Clover. The villains plan on using Nu-13 to copy Noel's trait as the Successor of the Azure to summon the Master Unit and are successful in doing so. The Imperator reveals her plan to the whole world and denounces her position, declaring herself "Hades: Izanami" and plans on creating a world of death with the souls of everyone on the planet, aided by Relius who opened every cauldron in the world, causing seithr to overflow and cause lots of casualties due to seithr poisoning, with their souls flying towards the Monolith of Ibukido. Their plan proves to be successful and the Master Unit is summoned.
With the assistance of Makoto and Tsubaki, Ragna manages to head to Izanami's location with Noel. Izanami then summons "Gigant: Takemikazuchi" from his four year slumber and plans on using it to destroy the Master Unit. Rachel uses the Tsukuyomi Unit to defend the Master Unit from any attacks. Jin meets up with the trio and the four of them attempt to stop Takemikazuchi before Kokonoe is forced to use Celica as a means of doing so. Hakumen arrives to give his assistance and faces off with Terumi and destroys him with his Time Killer technique. Nu merges with Takemikazuchi and engages Ragna, Jin, and Noel. Bang is successful in activating the Lynchpin and elminating the world's seithr. Jin departs to assist Hakumen on cutting the Ibukido Monolith and Ragna activates Blazblue to defeat Takemikazuchi. Ragna and Celica head inside the weapon's core to extract Nu and are successful.
Afterward, Ragna and Celica meet up with Jin, Noel, and Hakumen all of them succeed in thwarting the villain's plans. Izanami shows up, and proves immune to Hakumen's Time Killer. She then commands Phantom to teleport Hakumen away, and to deal with Jubei, during which she releases the limiters on Phantom, allowing Jubei to see her true identity. She then forcibly activates Ragna's Azure Grimoire, causing him to mutate into a monstrosity like the Black Beast. Noel activates her powers as the Godslayer, and she and Jin attempt to fend off Ragna, but are critically injured in the process. Izanami, with the collected souls from the Monolith, turns Takemikazuchi into a large sphere known as the "Embryo" and welcomes "Doomsday". The remaining heroes attempt to deal with all of the current problems while Ragna is MIA after he loses control.
Immediately after the events of Chronophantasma, Izanami tries to open the Master Unit, to find that it is "hollow".
Jin is on his sickbed after being treated, after his encounter with Ragna. After talking to Jin, Noel meets Rachel and asks her the same thing Jin asked: about a certain "gate" she saw in her memory when Ragna "devoured" both Jin and Noel, but she for some reason cannot remember much beyond that. Suddenly, they're teleported to Izanami's place. Izanami tricks Noel into using her powers to locate Ragna and then Izanami activates the Embryo, and everything suddenly turns black.
Ragna then wakes up at a certain forest near Kagutsuchi, devoid of his memories. He's forced to relieve the events of Calamity Trigger without any of his memories, including how he's a SS Class criminal.