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Over 100 years before the beginning of the story on an alternate world, giant humanoid creatures called Titans (巨人 Kyojin) suddenly appeared and nearly wiped out humanity, devouring them without remorse or reason and only targeting them; they completely ignore other wildlife. What remains of humanity now resides within three enormous curtain walls: the outermost is Wall Maria (ウォール・マリア Wōru Maria); the middle wall is Wall Rose (ウォール・ローゼ Wōru Rōze, sometimes pronounced like rosé) and the innermost is Wall Sheena (ウォール・シーナ Wōru Shīna, alt. "Wall Sina"). Inside these walls, humanity has lived in uneasy peace for one hundred years, many people growing up without ever having seen a Titan. This all changes when one day, a giant 60-meter (200 ft)-tall Colossus Titan mysteriously appears after a strike of lightning and breaches the outer wall (which is 50 meters tall) of the Shiganshina district, a town at Wall Maria, allowing the smaller Titans to invade the district. An Armored Titan smashes clean through the gate in Wall Maria, forcing humankind to abandon the land between Wall Maria and Wall Rose, evacuating the remaining population into the inner districts. The sudden influx of population causes turmoil and famine.

The Titans are giant humanoid figures ranging between 3 and 15 meters (10 and 50 ft) tall and are usually masculine in body structure but lack reproductive organs. Although they do not appear to need food, they instinctively attack and eat humans on sight; it is mentioned that they derive their energy from sunlight. Furthermore, the Titans also do not have a proper digestive tract; once they have eaten their fill of human prey, Titans will vomit large, slimy balls, derisively referred to as "hairballs", as shown in the episode where the squad retakes Trost. Finally, their skin is tough and difficult to penetrate, and they regenerate quickly from injuries, except for a weak spot at the nape of their neck.

Combating the Titans is the military, which is divided into three branches. Foremost in the story is the Survey Corps (調査兵団 Chōsa Heidan), which goes out into Titan territory to try to reclaim the land. The Survey Corps is heavily derided in society because of its apparently senseless high casualty rate and little sense of progress. The second and largest branch is the Garrison Regiment (駐屯兵団 Chūton Heidan), which guards the walls and the civilian populace. The third branch is the Military Police Brigade (憲兵団 Kenpeidan), which guards the royal family and lives relatively relaxed lives in the innermost wall, although this eventually results in fraud, corruption, and political subterfuge. The soldiers use a grappling, tethering system called Vertical Maneuvering Equipment (立体機動装置 Rittai Kidō Sōchi) that allows them to jump onto (and swing from) walls, trees, or nearby buildings to attack Titans with dual swords, and use gas canisters which to propel themselves. However, despite it being the soldiers' primary line of both offense and defense against the Titans, it is useless in open and flat terrain like fields.

It is later revealed that the Titans were created when Ymir Fritz found the "source of all organic life" and became the Founding Titan which gave her the ability to create and control the Titans. When Ymir Fritz died as a side effect of having the Titan ability for 13 years, her power was divided among nine who pass their abilities to another upon death to other Subjects of Ymir, also known the Eldian people. These Titans consist of the Founding Titan, the Attack Titan, the Colossal Titan, the Armored Titan, the Female Titan, the Beast Titan, the Jaw Titan, the Cart Titan, and the War Hammer Titan. The Founding Titan remained with the Fritz family as Eldia's 145th King Karl Fritz arranged for his country's fall by Marley and took a handful of his people to the island of Paradis where he used the countless Colossus Titans to make the walls and erased the memories of the Eldians he brought with him so that they did not know anything about the outside world, his descendants continuing his work while the Eldians in Marley suffer as second-class citizen with some being exiled to Paradis as the Titans that terrorize the island's residents.