This documentary follows a small number of British people with an incurable genetic disease called Prader-Willi syndrome.

Prader–Willi syndrome is a rare genetic disorder in which seven genes (or some subset thereof) on chromosome 15 (q 11-13) are deleted or unexpressed (chromosome 15q partial deletion) on the paternal chromosome.

It was first described in 1956 by Andrea Prader (1919-2001), Heinrich Willi (1900-1971), Alexis Labhart (1916), Andrew Ziegler, and Guido Fanconi of Switzerland. The incidence of PWS is between 1 in 25,000 and 1 in 10,000 live births. The paternal origin of the genetic material that is affected in the syndrome is important because the particular region of chromosome 15 involved is subject to parent of origin imprinting, meaning that for a number of genes in this region only one copy of the gene is expressed while the other is silenced through imprinting. For the genes affected in PWS it is the maternal copy that is usually expressed, while the paternal copy is silenced.

This means that while most people have a single working copy of these genes, people with PWS have no working copy. PWS has the sister syndrome Angelman syndrome in which maternally derived genetic material is affected in the same genetic region.