What is the plot of the game World of Warcraft

World of Warcraft features several plots, depending on how you look at it.

There is the everpresent Alliance - Horde tension, which leads to skirmishes and war. This leads to battlegrounds and contested zones where players of different factions fight each other.

Another storyline features the heroes and anti-heroes marching against evils of Onyxia, the Old God Ch'tun and eventually Kel'thuzad (classic).

Following this, the opening of the Dark Portal allows players to face Lady Vashj and Kael'thas, and eventually their master Illidan. Upon their defeat, they find out that Kael'thas has struck a deal with the demon lord Kil'jaeden, and attempts to summon him to Azeroth (The Burning Crusade).

As this happens, the Lich King has gathered his legions in Northrend, but we cannot face him until we have defeated his lieutenant Kel'thuzad. Meanwhile, Malygos has gone insane and wishes to take magic away from the Mortal races, even as a great evil slowly frees itself from its prison in Ulduar. The Old God Yogg-Saron, strengthened by the evils in Northrend, turns out to be behind much of the tragedy from the Old World. (Wrath of the Lich King)

Now comes the time of turmoil, when Deathwing the Destroyer and Ragnaros the Firelord break free from the earth and bring ruin to Azeroth. (Cataclysm, yet to come).