能看见「恋爱的光线」的大学生西条，爱上了正在探索恋爱的女孩东云——。正是因为能看见，所以才会觉得痛苦难受。前所未有的恋爱故事开始了。

大學生西條擁有看得見“戀之光”的特異體質，然而自己卻過著與戀愛絕緣的生活。直到某天，他對極力想探究“愛為何物”的文學少女東雲一見鍾情，開始了以討論“戀愛定義”為主題的交換日記。同時，眼見兩人愈走愈近，這也讓一直單戀西條的青梅竹馬開始有點不是滋味。另一方面，只喜歡搶別人男友的宿木，因為誤把西條當做北代的男友，就此開始對他展開猛烈攻勢。

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.

「恋をしている女性が光って見える」という特異体質の大学生・西条は、自身は恋愛とは無縁の学生生活を送っていた。ある日、彼は「恋というものを知りたい」という文学少女・東雲に一目ぼれし、恋の定義について語り合う交換日記を始める。

'사랑을 하는 여성에게는 빛이 보인다' 특이한 체질을 가진 대학생 사이조. 연애와는 거리가 먼 그는 어느 날, '사랑이라는 것을 알고 싶다'라고 말하는 문학소녀 시노노메에게 첫눈에 반해 '사랑의 정의'를 나누는 교환일기를 시작한다. 그들의 모습에 사이조를 짝사랑 해온 소꿉친구 키타시로의 마음은 술렁인다. 그들은 수천 년 동안 인류가 고민해 온 '사랑'을 풀 수 있을까? 그리고 각각의 사랑의 행방은 어떻게 될 것인가?