A female high school student has been friends with a boy named Rintarou since they were children. He is tall, intimidating in appearance, yet popular with others. She watches as he turns down another girl who confessed to him, thinking what a waste it is that he always rejects everyone. When she asks him why he refuses every confession, he gives a startling answer: because he loves her. This is the first time she has ever heard him say this, throwing her entire perception of their ten-year friendship into confusion.

Caught completely off guard by his sudden and direct confession, the protagonist feels a strange mix of shock and frustration. She cannot believe he would make such a declaration after all this time, and the situation makes her feel as if she has lost some unspoken competition. In response, rather than simply accepting or rejecting him, she decides to put him to the test. She resolves to evaluate whether this long-time friend is truly worthy of becoming her boyfriend, initiating a new and awkward phase in their relationship.

The story is set in a contemporary high school environment, where the two main characters navigate the shift from a decade of platonic friendship to a potential romance. Rintarou is characterized as a straightforward and pure-hearted young man who has harbored feelings for his friend for years, while the heroine is more reactive and flustered by the unexpected turn of events. The narrative follows her various attempts to test Rintarou's sincerity and suitability as a partner, leading to a series of comedic and romantic situations as they both redefine their connection to one another.