The show follows numerous forensic investigations (referred to as "cases" in the beginning of each episode) led by William J. Birnes and his team of experts: researcher and scuba diver Pat Uskert, mechanical engineer and MIT researcher Ted Acworth, and investigative biologist Jeff Tomlinson. In the second season, Tomlinson departed. In the third season, Acworth was replaced by mechanical engineer Kevin Cook.
In each episode, the team investigates reports of unidentified aerial phenomena in which they interview witnesses of close encounters with UFOs, USOs, and supposed extraterrestrial life. They also analyze any evidence collected such as photographs, video, or recovered physical material. They also conduct research with other investigators and scientists in the field in an attempt to find conclusive evidence that a report is real or a hoax. The show also investigates long-standing UFO cases such as the Roswell, New Mexico UFO crash incidentand other famous sightings throughout history.
More info: http://en.wikipedia.org/wiki/UFO_Hunters