This section contains a summary of Babylon 5's story arc (the 5-year plotline) up to, but not including, "Midnight on the Firing Line." 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.