The 4400 is a show about the titular number of people being abducted at different points over the 60 years prior to the start of the show, and then all showing up at once with no memory of what happened. They also haven’t aged a day since they were abducted and some of them are beginning to manifest strange abilities such as precognitive visions and psychokinesis.
The main protagonists are two NTAC (a fictional government organisation) agents who are trying to learn more about the 4400, as well as stopping some of them from getting hurt or committing crimes. One thing did bug me about this setup though — the female agent came from the Center For Disease Control (and indeed, much talk of the 4400 being like viruses goes on for the first half of the series) but acts like she came from the FBI and even indulges in a bit of suspect profiling. While I admit it’s not impossible that she would’ve learned this, it doesn’t seem like it’s her job to be doing it.
You also see things from the view of some of the 4400, such as a man who becomes a vigilante upon his return, or the black man from the 1950s who still remembers the prejudices of that time and doesn’t feel much has changed.
Aside from the alien abduction angle (and the lack of aging), this was Heroes before Heroes was conceived. The show develops the characters by showing how they’re coping with adapting to life in the 21st century (although one or two that we see disappeared in this century) and dealing with their families or lack of same. It’s the kind of show that gets you thinking about the possibilities and gets you developing your own theories before you know what happens. Some of my thoughts include: the number of people taken seems too precise to be random, and giving people superpowers would be a good idea for experimentation if you had the power to do it.