The story begins in the year 845, inside the walled city of Shiganshina, located on the outermost of three concentric walls (Wall Maria). Eren Yeager, along with his adoptive sister Mikasa Ackerman and their friend Armin Arlert, witnesses a Colossal Titan—a Titan far larger than any seen before—breaching Wall Maria. This catastrophic event leads to an invasion of Titans, forcing the surviving humans to retreat behind the inner walls.