(covers information from several alternate timelines)
Pe'Nar Makull was leader of a group of protestors who were opposed to the use of polaric ion energy, believing that it was unstable and a threat to their planet. He was a native of Makull's homeworld.
He planned to sabotage a local power plant. Kathryn Janeway and Tom Paris, who had beamed down to the planet that was in ruins, were transported back to the past prior to the devastation. They tried to stop the protesters, believing that this effort would itself lead to the destruction of the planet. Makull met Janeway and Paris at a protest at the plant which turned into a riot. After both were hurt at the riot, Makull took them to his house. Makull suspected the two were government infiltrators, and asked them questions about where they lived and why they were at the demonstration.
After it was obvious that he did not believe them, Janeway told Makull who they really were and that his attempt to damage the plant would destroy his world. He did not believe her. Makull followed through with his plan to sabotage the plant, taking Janeway and Paris with his group. At the plant, Janeway realized that an attempt by USS Voyager to rescue them had caused the destruction of the planet. After Janeway was able to stop the rescue attempt, she and Paris returned to the future and the planet survived. (VOY: "Time and Again")