Just outside Paris, inside a hi-tech vault, and encased in three vacuum-sealed bell jars, rests a small metallic cylinder about the diameter of a golf ball.

Just outside Paris, inside a hi-tech vault, and encased in three vacuum-sealed bell jars, rests a small metallic cylinder about the diameter of a golf ball.

It may not look like much, but it is one of the most important objects on the planet. It affects nearly every aspect of our lives including the food we eat, the cars we drive, even the medicines we take. It is the kilogramme, the base unit of mass in the International System of Units. This small hunk of metal is the object against which all others are measured. Yet over time, its mass has mysteriously eroded by the weight of an eyelash. A change that, unbeknownst to most, unleashed a crisis with potentially dire consequences.

Follows the ensuing high-stakes, two-year race to redefine the weight of the world, and tells the story of one of the most important objects on the planet.