I want to know what love is...

Saijo has an ability to see special glow around women who are in love. Kitashiro, his childhood friend, the only person that knows about his ability, has had a crush on him most of her life, but Saijo tells her that she doesn't emit light around him which leaves her at a loss. Meanwhile, their classmate Yadorigi, who has a habit of liking guys belonging to other women, mistakes Saijo as Kitashiro' s boyfriend and makes a move on him. One day, Saijo accidentally sees a notebook that belongs to Shinonome, a girl who is trying to explore the meaning of love. They start an exchange diary to discuss the topic, but Kitashiro feels uncomfortable watching them deepen their relationship. And when Yadorigi sees this, she decides that she too wants to start an exchange diary with them.