Monochromatic people (known as 'Monos) live in a world where they are unable to see colour and therefore view their surroundings through varying shades of grey, black and white. However, every Mono has a fated person (known as a 'Probe') who is able to trigger their grey-scale world into colour, inducing what is termed a 'Colour Rush'. Yeon Woo is a Mono. He’s aware that other colours do exist but has never experienced them for himself, nor is he interested in doing so. However, Yeon Woo’s life is forever changed when he meets Yoo Han, his Probe, and experiences his first Colour Rush. Terrified that he'll be obsessed with Yoo Han because of Yoo Han's ability to make him see colour, Yeon Woo tries desperately to avoid Yoo Han—but why does Yoo Han insist on drawing close to him?

Choi Yeon Woo is a high school student and a “mono” who sees the world only in dull, lifeless black and white. But for every mono, there is a “probe” – a kind of soulmate who allows the mono to see the world in vivid color. Yeon Woo’s probe was a boy named Yoo Han, and the two developed a beautiful friendship that blossomed into romance. But as their relationship intensified, mysterious goings-on suddenly got very unpleasant for Yeon Woo, who eventually found that Yoo Han had disappeared, with his mother also vanishing. Yeon Woo vows to do whatever it takes to track down his “probe” and his mother. A classmate named Se Hyun learns of Yeon Woo’s quest and resolves to help and protect him – as a close friendship develops between the two students. But Se Hyun hides a secret, and Yeon Woo could be about to discover the truth.