Attack on Titan Vol. 1 (manga)

Humankind is down to just a few thousand people who live in a city surrounded by three concentric walls. The walls protect them from their enemies, the ravenous giants known as the Titans. The Titans appear to have only one purpose: to consume humanity.

For one hundred years, what's left of mankind has lived in the city on earth, protected by walls that tower over even the Titans. Untouched by the Titans for a century, humanity has become complacent. But Eren Jaeger has had enough. While his fellow citizens are content to hide, Jaeger has the passion to take action to not only protect the city, but to learn what the Titans actually are. On his first mission he comes face to face with horrors beyond his imagination... and secrets from his own past that could shift the tides of war.