16-year-old Grace awakes one morning to find the sky leaden, the sun a huge ball of ash, the clouds like threatening rubble, and reports of unexplained accidents occurring on roads and rail. These are the hallmarks of an apocalyptic movie, but it quickly becomes apparent that everything, to the rest of her family, seems normal; Grace is one of only a handful of people in the country who are seeing the world in shades of grey.

Soon, however, the whole of society is in the grip of the Monochrome Effect, or 'greyout', which eliminates the ability of humans and animals to see colour. The greyout moves from person to person, but it isn't a transmissible disease: the effect on the optic nerve can be traced from microplastics in the ocean, the result of unchecked pollution, now in all water systems.

When Grace starts to experience intermittent 'colour episodes', she is asked to join a government-run study with other teens who have seen flashes of colour since the Monochrome Effect began. She is told that she will be helping find a cure; be part of something that could save the world. But the reality is much more sinister, complex and dangerous than she could ever have imagined - colour vision is now currency, and to those in power, worth the ultimate price...