At the center of our galaxy sits an object so dense that stars orbiting it whip around at speeds that make no sense unless something with millions of times the mass of the sun is hiding there. This film follows the astronomers hunting for proof that Sagittarius A*, the radio source at the Milky Way's core, is in fact a supermassive black hole. Infrared telescopes track individual stars looping through tight, fast orbits around an invisible point, and researchers explain how those orbits let them calculate the mass and density of whatever is pulling on them. The film moves outward from that local case to the bigger question the discovery raises: whether every galaxy, including our own, grows around a black hole engine, and what happens to a galaxy when that engine occasionally flares to life. Interviews with observational astronomers walk through the decades of data collection required to rule out other explanations, building toward the conclusion that the evidence now points one way.