This section contains a summary of Babylon 5's story arc (the 5-year plotline) up to, but not including, "Soul Hunter." Note that this is chock full of spoilers!
The pilot, "The Gathering," was set in the year 2257; Babylon 5 had recently come online. The last of the ambassadors, Kosh, arrived on the station, prompting an assassination attempt. Sinclair was framed for the attempt, but was eventually cleared when the real culprit -- a Minbari -- was discovered. Many mysteries were introduced: Sinclair's missing 24 hours on the Battle of the Line (with the announcement by the Minbari that "there is a hole in your mind"); the mystery of Kosh, seen only by the doctor and the telepath; and the disappearance or destruction of the previous four Babylon stations.
Season One began with "Midnight on the Firing Line," about 6 months later. Lt. Cmdr Susan Ivanova replaced Lt. Cmdr. Takashima as second-in-command; her dislike of the Psi Corps became evident when Talia Winters arrived. A Centauri agricultural outpost was attacked without provocation by the Narns. (In broad strokes, this episode portrayed the Narns as the "bad guys" and the Centauri as comic relief.) Luis Santiago was reelected President of the Earth Alliance.