Tobi Otogiri, a second-year student in middle school, doesn't spend much time with his peers. Instead, he mostly chats in secret with his pal Baku, a mysterious backpack he received the day he last saw his older brother. Or at least, Tobi thought his chats were secret, until a girl in his class named Ryuuko suddenly mentions Baku. Soon, strange things start happening at school, and the two of them realize the cause might be a creature only they can see... Thus begins an adventure blending reality and fantasy, human and non-human—a story about life eating life.