Monochromatic people (known as 'Mono's) live in a world where they are not able to see colours and, therefore, view their surroundings through varying shades of grey, black and white. However, every Mono has a fated person (known as 'Probe's) who is able to trigger their grey-scale world into colour, inducing what is termed as 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 Yu Han, his Probe, and experiences his first 'Colour Rush'. Terrified that he would be obsessed with Yu Han because of Yu Han's ability to make him see colours, Yeon Woo tries to avoid Yu Han, but why does Yu Han insist on drawing close to him?

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… Meanwhile, Yeon Woo is not alone – other monos are nearer than he might think, including a female mono who is looking for her own probe. And a TV producer is also on the scene, hoping to get to the bottom of the mono-probe mystery!