The story of Attack on Titan revolves around the adventures of Eren Yeager, his foster sister, Mikasa Ackerman, and their childhood friend, Armin Arlert. After the wall which protects their hometown of Shinganshina is breached by Titans, Eren watches in horror as one of them eats his mother. Vowing to kill all the Titans, Eren enlists in the military, along with his friends.
Five years later, the three cadet graduates are positioned in Trost District, one of the border towns jutting out of Wall Rose, when the Titans attack again. In the ensuing battle, Eren is eaten by one of the Titans before Armin’s eyes. A Titan later appears and begins fighting the other Titans while ignoring humans; the Titan is revealed to be Eren, who has developed the ability to transform into one. Though he is seen as a threat by some, he helps the military take back Trost District. After being placed on trial for being a danger to the humans, he is taken in by the Survey Corps’ Special Operations Squad, led by Captain Levi.
In an expedition to Shinganshina, the Scouts are attacked by a Female Titan who attempts to capture Eren. Although the Scouts are able to briefly capture the Female Titan, she breaks free and devastates Levi’s squad, forcing the expedition to retreat. Armin determines that the Female Titan is Annie, one of the cadets who taught Eren to fight, and devises a plan to capture her at Stohess. During this operation, collateral damage reveals that Titans reside in and make up the walls surrounding the human settlements.
Some of Eren’s friends are also revealed to be able to transform into Titans and had been sent as spies by an unknown party to find something called “The Coordinate”. It is later confirmed that the Coordinate is the power to control other Titans at will, and that Eren holds that power. The royal family and the Military Police track Eren and his friend Krista Lenz, who is actually Historia Reiss, the successor to the real king, Rod Reiss, as the current king is but a figurehead.
The Survey Corps, losing all will to follow the current government, stage a successful rebellion to overthrow the monarchy, culminating with the death of Rod by the hands of his own daughter, who becomes the new queen. It is then revealed that the Reiss Family was responsible for the creation of the walls 100 years ago using the Coordinate, which was stolen by Eren’s father Grisha, who then transferred it to Eren’s body by turning him into a Titan before allowing his son to devour him. Eren also obtains a new power that allows him to create massive and permanent structures that the Survey Corps intend to use in order to seal the breach in Shinganshina and reclaim Wall Maria.