Attack on Titan is a Japanese manga series written and illustrated by Hajime Isayama. The series began in Kodansha's Bessatsu Shonen Magazine on September 9, 2009, and has been collected into 22 tankobon volumes as of April 2017. It is set in a world where humanity lives in cities surrounded by enormous walls; a defense against the Titans, gigantic humanoids that eat humans seemingly without reason. The story initially centers on Eren Yeager and his childhood friends Mikasa Ackerman and Armin Arlert, who join the military to fight the Titans after their home town is invaded and Eren's mother is eaten. However, as the story progresses and the truths about the Titans are slowly revealed to the reader, the narrative shifts to encompass Historia Reiss, squad leader Levi, Eren's father Grisha, and other supporting characters.