Tetsuo Takahashi, a high school biology teacher, may appear to be your average everyday teacher, but beneath his gentle exterior lies something more unusual: his fascination with the "Ajin," also known as "Demi." Although these half-human, half-monster beings have integrated into human society, Takahashi believes that much about them will remain unknown unless he interacts with them firsthand. Demi-chan wa Kataritai recounts Takahashi's daily routine at Shibasaki High School with his three Demi students: Hikari Takanashi, an active vampire; Kyouko Machi, a kind dullahan; and Yuki Kusakabe, the bashful snow lady. Takahashi also encounters fellow instructor Sakie Satou, a succubus with an aversion to males, along the way. Takahashi chooses to undertake informal conversations with the Demi in order to achieve his aim of knowing more about the Demi.