10. THE 4400 (USA, 2004-2007)
Mystery: Who are the 4400 and why have they landed on Earth?
In the pilot episode, a comet makes its way towards Earth and lands on the planet without causing any real damage. It turns out the comet is carrying 4400 humans who were thought to have been either killed or missing over the past 58 years. None of them have aged a day since they disappeared, and some of them even return with enhanced abilities, like precognition and telepathy.
As the various members of the 4400 attempt to move on with their lives, NTAC (National Threat Assessment Command) agents Tom Baldwin (Joel Gretsch) and Diana Skouris (Jacqueline McKenzie) investigate their mysterious reappearance. Were they abducted by aliens? Are they, in fact, aliens? Is there some other force at work? Without spoiling anything, let's just say that, at the end of season one, the series throws an exceedingly clever and unexpected twist at viewers.