Years are numbered in a single calendar, written here as "Year 845" and so on. The present of the game is Year 850, after the 57th Exterior Scouting Mission and before the Assault on Stohess. The three main protagonists were all born in Year 835, so they are ten years old in Year 845 and fifteen years old in Year 850. A cadet enlists at about age twelve and graduates after three years of training, which places the 104th Cadet Corps' enlistment in Year 847 and its graduation in Year 850.
The people inside the Walls believe that humanity was driven almost to extinction by the Titans about a century ago, that the surviving humans built three Walls and have lived behind them since, and that nothing but Titans lives outside. Public records state that the Titans first appeared roughly 107 years before Year 850. No one in the Walls knows the Walls are made of Titans, that the Titans are transformed Eldians, or that other nations exist. The king, who is a figurehead, is believed to be the rightful sovereign, and the Church of the Walls (the Wall Cult) holds that the Walls are sacred and that no one may investigate their nature. The real royal blood and real knowledge sit with a hidden Reiss family and a small group around the government.
The military has three branches. The Survey Corps ventures outside the Walls on expeditions to study and fight Titans. It suffers extreme casualties and is regarded by many as futile and wasteful. The Garrison guards the Walls and the gates and operates the Wall cannons. The Military Police maintains order inside the Walls, protects the king, and recruits only from the top ten graduates of each Cadet Corps class. The Cadet Corps trains young recruits for three years before they join one of those branches. The Military Police is regarded as the most desirable branch because its members live in the Inner District of Wall Sina and are far from the Titans, and it is also regarded as the most corrupt.
In Year 835 Eren Yeager, Mikasa Ackerman, and Armin Arlert are born. The three grow up in Shiganshina District, the southernmost district of Wall Maria, at the border of the Walls and nearest to the Titans' reach. Eren's father Grisha is the district's physician, and his mother Carla runs the household. Mikasa is taken in by the Yeagers after her parents are killed by kidnappers, an event in which Eren's first real act of violence occurs. Armin lives with his grandfather and reads obsessively about the world outside the Walls. In these years Eren grows convinced that humanity lives like livestock in a pen and develops a desire to see the outside world.
In Year 845 the Colossal Titan, a skinless Titan about 60 meters tall and far larger than any known Titan, appears at the Shiganshina gate and kicks it open. The Armored Titan breaks the inner gate of Wall Maria. Titans pour into Shiganshina. Eren's mother Carla is trapped under her collapsed house; the family friend Hannes, a Garrison soldier, panics and fails to save her, and she is eaten by a Titan in front of Eren. Eren, Mikasa, and Armin escape by boat with the other refugees. In the days after, Eren's father Grisha, who had been away, returns, injects Eren with a serum, gives him the key to the basement of their home, and then vanishes. Eren does not recall this. The Titans that entered Wall Maria are not driven out, and the whole of Wall Maria, with its farmland, towns, and population, becomes uninhabitable. The Garrison pulls back and holds Wall Rose.
Wall Maria's survivors are crammed into Wall Rose and Wall Sina. Food shortages follow. In Year 846 the government, to reduce the refugee burden and attempt to reclaim land, sends a very large number of the displaced to retake Wall Maria. The attempt fails with massive loss of life. Armin's grandfather is among the dead. The event is remembered as an act of government desperation and leaves a hard memory among the refugees.
In Year 847 Eren, Mikasa, and Armin enlist in the 104th Cadet Corps, along with the Southern Division's other recruits, including Reiner Braun, Bertolt Hoover, and Annie Leonhart, who have infiltrated the corps under cover. Keith Shadis, a former Survey Corps commander who resigned his post, is the head instructor of the Southern Division. The corps trains for three years. Eren nearly fails the first test, the balancing aptitude test on ODM gear, because Shadis secretly tampered with his equipment. Eren's equipment is replaced and he passes. Over three years the recruits train in ODM gear, hand-to-hand combat, horseback riding, and tactical theory. Mikasa finishes first in the class. The other nine in the top ten, in order, are Reiner, Bertolt, Annie, Eren, Jean Kirstein, Marco Bott, Connie Springer, Sasha Blouse, and Krista Lenz. Ymir, one of the strongest cadets, deliberately underperforms in the last weeks to protect Krista's place in the top ten.
Around the time of the 104th's graduation, the Colossal Titan appears again at the outer gate of Trost District in Wall Rose, the first Titan appearance within the Walls in five years, and kicks the gate open. Titans flood Trost. The Garrison and the newly graduating Cadet Corps are thrown into the defense. Many of the 104th are killed, including Marco Bott. During the fighting, Armin is swallowed by a Titan, and Eren, trying to save him, is also swallowed. Eren emerges transformed into a Titan with intelligence, the Attack Titan, and fights other Titans. Garrison soldiers, believing him a threat, nearly destroy him. Garrison Commander Dot Pyxis hears of the Titan-Eren and assesses that he can be used. A plan is formed to escort the Eren-Titan to the damaged gate to seal it with a giant boulder. Eren, with Mikasa guarding him, does so. Trost is saved and all Titans inside are exterminated. This is the first time humanity wins against the Titans within the Walls.
Eren is arrested as a Titan. A military tribunal weighs whether to execute him or give him to the Survey Corps. Survey Corps Commander Erwin Smith argues for taking him, and Captain Levi, who is regarded as humanity's strongest soldier, demonstrates through a violent display that Eren can be restrained. Eren is placed in the Survey Corps' custody under Levi's supervision. A special unit called the Special Operations Squad (Levi's Squad), made of Petra Ral, Oluo Bozado, Eld Jinn, and Gunther Schultz, is formed to guard him. The cadets of the 104th who rank in the top ten choose their branches. Eight of the ten join the Survey Corps; Annie alone applies to the Military Police; Marco Bott is dead. None choose the Garrison. Eren himself joins the Survey Corps.
The Survey Corps launches its 57th expedition outside the Walls under Erwin's long-range scouting formation, a wide arrangement of units designed to detect and relay information on Titan movement. The mission's secret objective is to move Eren toward Shiganshina. An unusual Titan, a feminine Titan with intelligence, the Female Titan, attacks the formation. It specifically hunts Eren, killing many scouts. The Female Titan enters the Forest of Giant Trees, where Levi's Special Operations Squad is killed. Eren transforms in an attempt to fight and fails when overwhelmed by the Female Titan. The Female Titan captures Eren, bites into the Titan-Eren's nape, and removes him from his Titan, then flees with him. Levi pursues, rescues Eren, and the expedition returns to the Walls, having lost many soldiers. The Female Titan escapes.
This ledger states what each group knows at the present moment: after the 57th Exterior Scouting Mission, but before the Assault on Stohess. Anything not listed is unknown.
The general population of the Walls knows that the Colossal Titan appeared at Trost; that a Titan called "Eren Yeager" helped them; and that the Survey Corps lost many soldiers outside. They do not know the identity of the Female Titan. The Military Police and the central government know that Eren can transform and that the Survey Corps suffered heavy losses on the 57th expedition, but they do not yet know Annie's identity as a Titan. The Survey Corps, led by Erwin Smith, knows that Eren can transform and that the Female Titan specifically hunted him during the 57th expedition. Erwin and Armin suspect that the Female Titan is someone among the 104th Cadet Corps; Armin suspects Annie Leonhart, but this has not yet been confirmed publicly or acted upon in Stohess. Erwin suspects that other Titan shifters are inside the military; he knows that more than one exists, but no one in the Survey Corps knows that Reiner, Bertolt, or Ymir are shifters. Eren Yeager knows that he can transform into a Titan. He does not know that he holds the Founding Titan, does not know what his father did, and has no idea what is in the basement of his home. Armin Arlert is the best strategic mind in the corps and has begun to suspect that the Walls hold secrets. Mikasa Ackerman is bound to Eren. The Reiss family and the central government know the true history and conceal it. The Ackerman clan's powers are unknown to the Survey Corps; Levi himself does not know why he is so strong.
The hidden Marleyan agents are Reiner Braun, Bertolt Hoover, and Annie Leonhart. They remain inside the military and are unsuspected as enemy agents. Annie is still free and has not been exposed as the Female Titan. Ymir, a Jaw Titan shifter, is serving among the 104th without anyone knowing. The Beast Titan (Zeke Yeager) and Marley itself lie wholly outside the world's knowledge.