God Eater takes place in a post apocalyptic, near future where the earth is plagued by altered beasts known as "Aragami" (Bewildered God/Fallen God). Aragami have an insatiable appetite, take on the form of what they eat, evolve, and learn at an exponential rate. They are made up of things called "oracle cells" that rapidly regenerate preventing regular weapons from harming them.
A biochemical corporation called "Fenrir" was formed after it created the only weapon capable of killing the Aragami called a "God Arc". Warriors who face Aragami are called "God Eaters". All God Eaters wear special armlets that inject them with oracle cells to prevent their own weapons from devouring them. Every God Arc is specially matched to its wielder and cannot be handled by other God Eaters who would risk being devoured by it.
Occasionally an armlet may malfunction and slowly turn a human into an Aragami. A human turned into an Aragami can only be slain with their own weapon. Aragami can also take on the form of a human and in doing so become friendly to humanity, although they require time and teaching to act, speak, and understand human ways.