When 15-year-old Greta Thunberg skipped school to sit on the pavement outside the Swedish Parliament, holding a sign that said "School Strike for the Climate," director Nathan Grossman was there filming her in those earliest days. He was there when she traveled to European capitals — by electric car or train to discourage emissions from planes, and accompanied by her protective father — speaking to ever-growing crowds and calling for aggressive action to protect the planet.