Cameras continue to follow the lives of the Parkinson's sufferers who agreed to take part in a clinical trial testing a drug that could be the first to halt progression of, or even reverse, their condition. The surgery and infusions are medical firsts, offering a whole new way of delivering drugs to the brain. So far, the volunteers have been split into two groups, with half receiving GDNF (glial-cell derived neurotrophic factor - a naturally occurring protein found in the brain) and half just a placebo. Assessments show significant improvement in some. Then at the halfway stage, all the volunteers start receiving GDNF. Will the trial prove a success?