Very little is known about the Titans and their origins. All that is known is that they are giant humanoids that vary between 3 and 15 meters in height, and they instinctively attack and devour ordinary humans on sight. However, they apparently do not require food for sustenance, as they do not prey on other animals, surviving the century beyond the walls even with the long absence of humans, and apparently gain their energy directly from sunlight. They have hard skin and regenerative abilities, and can only be killed by a deep incision at the base of the neck. This weakness led to the development of the Vertical Maneuvering Equipment, allowing humans to navigate quickly in a three-dimensional space. Although it permits great mobility to a skilled user, it carries a large risk of overtaxing the muscles, requiring extensive physical conditioning and agility.
The series follows the story of Eren Yeager, his adopted sister Mikasa Ackerman, and their friend Armin Arlert. After the outermost Wall is breached by Titans, including the 60-meter tall skinless Colossus Titan and the abnormally intelligent Armored Titan, and his mother dying in the resulting destruction, Eren vows revenge against the Titans and later enlists in the military, accompanied by Mikasa and Armin. Years later during his first battle against the Titans, Eren discovers that he has the ability to transform into a Titan at will. Seen as a threat to mankind by some, and as a ray of hope by others, Eren joins his companions in a desperate fight to protect what remains of their society while looking for answers to the mysteries surrounding the Titans, the Walls, and his own existence as well.