A comet is due to pass by Earth, but its trajectory changes to a collision course, causing widespread panic. When the comet appears to slow down — as if it's coming in for a landing — 4,400 people are deposited in an explosion over Highland Beach.
The returnees — "the 4400" — have been missing from months to years. None have any memory of where they've been, none have aged, and a small number are beginning to manifest paranormal abilities. Tom Baldwin and Diana Skouris, two Homeland Security agents, are appointed to find out what happened to the 4400.