Attack on Titan is a Japanese manga series written and illustrated by Hajime Isayama. The series began serialization in Kodansha's Bessatsu Shōnen Magazine magazine from September, 2009 and is published in North America by Kodansha Comics USA. A spin-off light novel began release from December 2011 and has also received a manga adaptation. A television anime adaptation produced by Wit Studio and Production I.G aired in Japan on MBS between April and September 2013. A live action film adaptation is also in production. Four video game adaptations developed by Nitroplus staffers in collaboration with Production I.G were announced to be released as bonus content for the third and sixth volumes of the Blu-ray Disc release of the anime. Another game is being developed by Spike Chunsoft for the Nintendo 3DS. The manga series has become a commercial success, selling over 20 million volumes as of mid-2013. The release of the anime also saw a boost in the series' popularity with several writers receiving critical acclaim for its atmosphere and story. Although it also gained fame in neighboring Asian countries, the series's themes have been a subject of controversy. One hundred years prior to the beginning of the story, giant humanoid creatures called Titans have forced humanity to retreat into a system of concentric walls known as Wall Maria, Wall Rosé, and Wall Sina). Inside these walls, humanity lived in peace for one hundred years, but a sudden attack by a gigantic sixty-meter Titan and a smaller Titan covered in armor plates caused the outermost wall to be breached, forcing mankind to abandon the land between Wall Maria and Wall Rosé, resulting in turmoil and famine. Little is known about the Titans; giant creatures ranging from 3-15 meters tall, which instinctively attack and devour ordinary humans on sight. However, they apparently do not require meat as sustenance, as they do not prey on other animals, 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 premits 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.