http://www.blizzard.com/diablo3/ Just go straight to the official site — it’s seriously exciting!
Diablo 3 is no longer just a role-playing game; Blizzard has blended in a lot of competitive-gaming flavor. Go take a look!
Here’s a quick summary of the story:
Two decades have passed since the demonic denizens, Diablo, Mephisto, and Baal, wandered the world of Sanctuary in a vicious rampage to shackle humanity into unholy slavery. Yet for those who battled the Prime Evils directly, the memory fades slowly and the wounds of the soul still burn. When Deckard Cain returns to the ruins of Tristram’s Cathedral seeking clues to new stirrings of evil, a comet from the heavens strikes the very ground where Diablo once entered the world. The comet carries a dark omen in its fiery being and it calls the heroes of Sanctuary to defend the mortal world against the rising powers of the Burning Hells – and even the failing luminaries of the High Heavens itself.
More than twenty years have passed since the demons Diablo, Mephisto, and Baal threw the world of Sanctuary into chaos, letting evil run rampant and humanity be enslaved as its minions. Although some did successfully defeat the demon lords, and the memory of the disaster slowly fades, the wounds on the soul still ache.
When Deckard Cain returns to the ruins of Tristram’s Cathedral to seek clues to the new evils, a comet from the heavens strikes the ground — and the place it lands is exactly where Diablo once entered this world. The comet’s fiery being brings a dark omen. It also draws the heroes of Sanctuary, who must defend the mortal world and urgently fight the surging power of the Burning Hells — even the elders of the High Heavens themselves come to join the battle. (Brief translation by Mousebomb)
异次元の世界 has a detailed report