Since scientists began measuring sunlight in the 1950s, they have found a steady decline in the amount reaching the Earth's surface, in some regions by as much as 20 percent. This film follows the researchers who traced the cause to particles thrown into the atmosphere by industrial pollution, aircraft contrails, and vehicle exhaust, which reflect sunlight back into space and seed clouds that block even more. Interviews with atmospheric scientists, including researchers who examined sunlight records from Israel and pan evaporation data from around the world, lay out how the discovery emerged almost by accident from datasets meant to measure something else. The film explains why the three-day grounding of aircraft after September 11, 2001 gave scientists an unplanned experiment in what skies look like without contrails. The real concern driving the film is what happens if dimming has been masking the full extent of global warming, meaning that cleaning up pollution without addressing greenhouse gases could unmask a much faster temperature rise than currently measured.