Attack on Titan is a Japanese manga series. The series began serialization in Kodansha's Bessatsu Magazine on September 9, 2009, and has been collected into 12 volumes as of December 9, 2013. It is set in a world where humanity lives inside cities surrounded by enormous walls due to the Titans, gigantic humanoid creatures who devour humans seemingly without reason. The story centers on Eren Yeager, his adoptive sister Mikasa Ackerman, and their friend Armin Arlert, whose lives are changed forever after the appearance of a colossal titan brings about the destruction of their home town and the death of Eren's mother. Vowing revenge and to reclaim the world from the Titans, Eren, Mikasa and Armin join the Survey Corps, an elite group of soldiers who fight Titans outside the walls.