The 4400
The National Threat Assessment Command (NTAC), a division of the Department of Homeland Security, is in charge of dealing with the return of the 4400. Dennis Ryland is the head of NTAC. Ryland assigns Tom Baldwin and Diana Skouris as the lead team to investigate the 4400. In season 2, Ryland goes to Washington and is replaced by Nina Jarvis. In Season 4 Meghan Doyle takes over as the head of NTAC.
Many of the returnees have trouble getting their lives back on track after being gone from the world for years. More significantly, a small number of the returnees begin to manifest paranormal abilities, such as telekinesis, telepathy and precognition, as well as other "gifts". For example, in the pilot, Shawn Farrell manifests an ability to heal the broken neck of a dead bird, bringing it back to life. In addition, one of the 4400, Lily Moore, has become pregnant between her disappearance and return.
The first-season finale, "White Light", reveals that the 4400 were abducted not by aliens, but by humans from the Earth's future, that Kyle Baldwin was to be their "messenger", and that they were returned to avert a catastrophe.
By the second season, it is revealed that all 4400 have a fictional neurotransmitter, promicin, in their brains, which gives them their powers. The government, afraid of what this large group would do with such power, had secretly been dosing all of the 4400 with a promicin inhibitor, which had worked on most, but not all, of the 4400. One of the inhibitor's side effects is a potentially fatal immune deficiency. The inhibitor is ultimately removed from the 4400 by a dose of promicin extracted by Kevin Burkhoff from the blood of the infant Isabelle, who was never given the inhibitor.
At the beginning of the third season, the Nova Group, a terrorist faction made up of 4400s, emerges. Originally formed as a "defensive" group in the aftermath of the promicin-inhibitor scandal, the Nova Group eventually carries out numerous terrorist attacks against the government and NTAC. The group is responsible for many terrorist attacks including the assassination of the men involved with the promicin-inhibitor conspiracy; the attempted assassination of Ryland; the framing of Baldwin for murder; and the driving of another person to insanity.
During the third and fourth season, it is revealed that only a certain faction from the future wants to see history changed. Another faction, which prefers the status quo, opposes the 4400, and has sent their own operatives, including Isabelle Tyler and "the Marked", into the past. The exact motives of both factions have not yet been revealed.
Eventually, Jordan Collier, a returnee who declares himself the savior of humanity, makes promicin shots available to the general public. However, only half of the human population can actually tolerate promicin, and thus develop superhuman abilities, while the other half die upon taking the shot. Although the government outlaws promicin use, thousands of previously ordinary people have developed superhuman abilities, severely complicating NTAC's task. Collier later annexes a part of Seattle and transforms it into "Promise City", a self-proclaimed paradise open to all people with superhuman abilities. The US government attempts to reclaim Promise City but meets with little success.
At the conclusion of the series, Danny Farell's uncontrolled ability exposed some of the residents of Seattle to promicin, resulting in about 9,000 deaths and as many newly empowered humans, while at the same time forcing the remains of NTAC (now themselves mostly promicin-positive) to ask Jordan Collier and his followers (as the only group immune to the 50% chance of death from exposure) to become the de facto government of Seattle. The series ended with a cliff-hanger, with Collier pledging to build the future he had promised, while the government watches uneasily as Collier's militia remains in control of Seattle, now known as Promise City.
=== Ripple effect ===
Early in the first season, theorists at NTAC suspect that the 4400s have been placed in the timeline with their enhanced abilities for a purpose, to cause some change that could propagate and redirect the course of history. An example of this was when a 4400 named Carl Morrissey attempted to use his enhanced strength and reflexes to fight crime in his neighborhood park. Although Morrissey was killed by a group of thugs, his death inspired others to become involved in various neighborhood clean up projects.
Another example of this effect at work came during the season two episode "Wake up Call". In that episode a mentally ill returnee uses mind control powers to force the other residents of her mental hospital to build an unusual machine. When the machine is activated it cures fellow patient Kevin Burkhoff of his crippling schizophrenia. When NTAC looks into Burkhoff's background they discover that prior to falling ill he was a brilliant neuroscientist who was working on gene therapies which could end up being the basis of the technology which created the 4400.
murder