Matt Baker introduces a special episode that comes from all four corners of the UK. Ben Nevis, Slieve Donard, Yr Wyddfa and Scafell Pike – the highest peaks of our four nations are the breathtaking backdrop for a massive art installation. Hundreds of volunteers, or 'lumenators', with specially designed bespoke lights, head for the peaks for this ambitious project celebrating access to and custodianship of the countryside. With a presenter on each of the mountains, we go behind the scenes to discover the vision behind the project, hear the stories of the volunteers and find out how the lighting and drone teams tackled some testing challenges. It all culminates with a 15-minute spectacle marking the finale of the Green Space, Dark Skies project.