The season features the conclusion of several plot-lines introduced in season two, while also introducing several new plot elements. Major plot arcs include an elaborate conspiracy being discovered when an alien autopsy video is acquired by Mulder, Scully's search for the killer of her sister, and the mystery surrounding X (Steven Williams). Pivotal characters such as the First Elder (Don S. Williams) and the alien virus black oil were first introduced in this season. In addition, the season features a wide variety of "Monster-of-the-Week" episodes, stand-alone stories not of influence to the wider mythology of the series.