The first in a two-part documentary following a group of volunteers with Parkinson's disease as they take part in a ground-breaking medical trial to test a new drug, run by doctors in Bristol. Filmed over six years, the first step is for all 42 volunteers to have surgery, implanting a device into the side of the head that allows the delivery of the drug into a deep part of the brain. The surgery and infusions that follow are medical firsts and the trial offers hope to the estimated 10 million affected globally.