At the dawn of the twentieth century, visionary inventor Nikola Tesla struggles to gain the American public's confidence in the face of a withering public relations assault waged by his rival, Thomas Edison. To make matters worse, Tesla's Niagara Falls hydroelectric facility falls victim to an act of sabotage, which leads to a man's death. The only clue is a strange, floating green sphere, the presence of which draws the Doctor's attention -- just in time to save Tesla from an otherworldly attack. Is Edison behind the mysterious goings-on? And how are these events connected to the signals Tesla has detected from outer space?