From the liner notes

Over the last century, thousands of people have gone missing. Suddenly and inexplicably, 4400 missing people are returned, all at once, exactly as they were on the day they vanished. Unclear what this world-altering event means, the government investigates the 4400 to piece together where they've been and why they've been returned. It quickly becomes apparent that their presence will change the human race in ways no one could have foreseen.