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?.