The story of Attack on Titan is set in a world where humanity is on the brink of extinction due to giant humanoid creatures known as Titans that devour humans. These Titans have appeared mysteriously, and humanity has retreated to a walled city for protection. The main protagonist, Eren Yeager, along with his friends Mikasa Ackerman and Armin Arlert, witnesses the destruction of their home city by a colossal Titan and sets out to fight back, joining the military to protect humanity.

As the series progresses, Eren and his friends discover dark secrets about the Titans, the history of their world, and their own origins. The story delves into political intrigue, moral dilemmas, and the nature of freedom, identity, and survival.

Karl brought his remaining citizens to Paradis Island and used countless Colossal Titans to establish a walled city consisting of three layers of walls: the outermost was Wall Maria the middle was called Wall Rose, sometimes spelled rosé and the deepest was called Wall Shina, “Wall Sina”.

Karl then used the power of the Pioneer Titan to erase the memories of most Eldians. Those who were not affected by the power were bribed into nobility or made outcasts like the Ackerman family who served the Fritz family. Because of the power of the Pioneer Titan, Karl influenced his descendants to continue his task of ruling the Eldians within the walls by promoting fear of attack by the Titan, whose true identity was an altered Eldian exiled by Marley on Paradis to terrorize the islanders.

Because Karl used the power of the Pioneer Titans to continue his ruse to influence his descendants, the Eldians on the island of Paradis believed that they were the only human beings left in the world and held the Titans responsible for their terror. Unlike those with Titan powers, the Pure Titans have almost no consciousness as they attack and eat humans on sight instinctively. Their skin is hard and impenetrable, and their bodies regenerate quickly from any wounds they receive, except for a spot at the nape of their neck-attacking that part is the only way to kill them. The Eldians of Paradis developed their own military branches to fight against the Titans. The foremost branch was the Inquisitor Squad which voluntarily attempted to reclaim territory taken by the Titans and was highly derided among the people for having a high casualty rate and making little progress in carrying out missions. The second and largest branch is the Standby Force, which keeps the walls and civilian population safe. The third branch is the Military Police, who guard the royal family and live a relatively relaxed life in the innermost regions of the wall, though this eventually leads them to engage in fraud, corruption, and political subterfuge. The soldiers use an iron grasping device as well as a body harness called the Three-Dimensional Maneuvering Device. It allows them to jump onto (and swing from) nearby walls, trees, or buildings to attack Titans with dual swords, as well as use gas cylinders to propel themselves. However, while the device became the warrior's primary combat power of both attack and defense against the Titans, it was useless in open, flat terrain such as fields.