This section contains a summary of Babylon 5's story arc (the 5-year plotline) up to, but not including, "Acts of Sacrifice." Note that this is chock full of spoilers!
In "The Geometry of Shadows," Refa, a high-ranking Centauri noble, asked Londo to pledge his help in seizing the throne at some point.
In "A Distant Star," Lt. Keffer saw a Shadow ship in hyperspace and vowed to find out what it was.
"A Spider in the Web" revealed a mysterious "Bureau 13" operating out of the San Diego wastelands. An assassin, controlled by the Bureau, attempted to thwart a peaceful settlement of the situation on Mars. Scanning his mind, Talia saw a Psi Cop present at the operation that programmed the assassin, but she didn't tell anybody else. Sheridan's interest in conspiracy theories was revealed as he attempted to solve the puzzle, and the existence of a plant among the B5 crew, code-named Control, was hinted at.
In "Soul Mates," Londo's three wives came to the station; one, who knows G'Kar, tried to murder him. He divorced her and one of the others. Talia's ex-husband, from an arranged marriage courtesy of the Corps, tried to woo her back.
In "A Race Through Dark Places," Bester returned to Babylon 5. He believed an underground railroad was smuggling unregistered telepaths outside of Psi-Corps' control. Talia was captured by the rogue telepaths and told gruesome stories of the Corps' true nature: forced breeding, murder, and coercion. She and the others tricked Bester into thinking the railroad was stopped. Dr. Franklin was revealed as one of the railroad's participants. Talia discovered another part of Ironheart's "gift" -- she is apparently now immune to telepathic scans.
At Refa's behest, as the Centauri emperor lay dying, Londo asked Morden to destroy a major Narn colony in "The Coming of Shadows." The Narns, believing the Centauri military responsible, declared war. Sinclair sent a message to Garibaldi via a network of "rangers," spies, a group he apparently heads.
In "All Alone in the Night," Delenn was removed from the Grey Council, and a member of the warrior caste replaced her. We learned that Sheridan had been secretly evaluating the loyalty of the station's command staff, and is in league with a group (headed by General Hague, a member of the Joint Chiefs of Staff) out to expose what they believe is an attempt by the Psi-Corps and others to control the government. Of special note is a densely-packed dream sequence in which Sheridan is visited by Ambassador Kosh.