[5] Yeon Woo is a monochrome person (also known as a "mono"), who sees the world only through different shades of gray, white and black due to his total color blindness (achromatopsia).

Choi Yeon Woo is a high school student and a "mono", a person who sees the world through different shades of gray, white, dull and lifeless black.

Yeon Woo's probe is a boy named Yoo Han, and the two developed a beautiful friendship that blossomed into romance.

As their relationship intensifies, mysterious events unfold that suddenly become very unpleasant for Yeon Woo, who eventually finds out that Yoo Han is missing.

A classmate named Se Hyun learns of Yeon Woo's search and decides to help and protect him, and a close friendship develops between the two students.