The quests in The Burning Crusade are generally set in continuity with original lore quests, meaning that in relation to the timeline, the expansion is the direct sequel to the original World of Warcraft. Some quests set in Outland, especially those that are related to Illidan, mention a five-year hiatus in the storyline, which suggests a one-year interval between The Burning Crusade and the original game. This is unconfirmed however, since there never was any real statement that the events of Warcraft III: The Frozen Throne take place one year after Reign of Chaos. Regardless, it is generally agreed upon that there was a four year break between Warcraft III and the start of World of Warcraft.
The expansion introduces Outland, the former world of Draenor, and numerous quests essentially allowing players to explore the world on their own. The main storyline points to Illidan the Betrayer and what happened to him during the last four years. Players can also complete quests that involve either aiding Illidan or making preparations for his downfall.
In addition, there are many links between the former world of Draenor and the current Outland, such as the opening of the Dark Portal by Medivh, which is relived in the Caverns of Time, and various quest chains which provide insight into the history of the old Orcish shamans and their relationship with Burning Legion long before the First War.
The Burning Crusade also centers on the two new playable races and their affiliation to the game’s storyline. Many plot holes are filled with quests that align the blood elves with the Horde and the draenei with the Alliance. Players still must complete quests in the old world of Azeroth (world) to fully understand the lore related to Outland.