As the title suggests, the center of the story is a man named Michael Korvac — an ordinary human who lives in the year 3000 in a universe separate from the normal Marvel Universe (Earth-691 instead of Earth-616).
After conflicts with the Guardians of the Galaxy from Earth-691 (the aforementioned group of characters), he is ultimately able to travel across time and space to the normal Marvel Universe.
Orchestrated by Jim Shooter, David Michelinie, George Pérez and David Wenzel, The Korvac Saga was a major arc that played out in Marvel Comics in the late 1970s.
Stealing the power of the cosmic entity Galactus and gaining god-like power, he makes plans to reshape the world — leaving only The Avengers and the pursuing Guardians of Galaxy to stop him.