It has been nearly fifteen years since the war between the orcs and humans ended. An uneasy peace settled over the land while, for years, the drums of war were silent. Yet the kingdoms of men grew complacent in their victory - and slowly, the defeated orcish clans regrouped under the banner of a new visionary leader.
Now a darker shadow has fallen over the world, threatening to extinguish all life - all hope. The drums of war play upon the winds once again - rising urgently towards the inevitable hour when the skies will rain fire - and the world will tremble before the coming of the Burning Legion.
The Day of Judgment has come...
Thrall wakes up in the middle of the night after having a strange nightmare. At dawn of the following morning he is greeted by the same shape-shifting crow that he saw in his nightmare, who orders Thrall to follow him. Intrigued, the warchief leaves his encampment with three of his grunts. He fights his way through gnolls, murlocs, golems and trolls before he reaches his destination, where the crowman prophet tells him that he must leave for the distant land of Kalimdor beyond the sea in order to save the world from the approaching demon threat.
Three days later, as Thrall is waiting for the orc clans to assemble, he learns that his comrade Grom Hellscream has been captured by humans. Thrall fights the humans, rescues Grom, and uses the orphaned human ships to take the Horde across the sea. Meanwhile, the prophet flies to the human city of Lordaeron with the hope of convincing King Terenas to follow the orcish steps, but the humans are too proud and they do not heed his warning.
As the prophet fails to convince the mages of Dalaran too, the king sends his son Arthas to defend Strahnbrad against the orcish attacks from the Blackrock clan where he finds out that the orcs are sacrificing humans in order to summon demons. And he gets very angry! This is how trouble usually starts. Prince Arthas decides that he can do it all by himself even as he is attacked by skeleton warriors, against better judgement from the celebrated paladin Uther the Lightbringer.
Arthas and his childhood friend Jaina Proudmoore discover that a plague is causing the kingdom’s people to turn into the undead after eating plagued grain as planned by the necromancer Kel’Thuzad. When Arthas sees that the whole city of Stratholme has been infected, he orders it purged against Uther’s refusal to do so. The angst starts taking hold of Arthas, as he banishes the paladin from his army, and at the same time loses the support of Jaina.
As Arthas destroys the undead city that was once populated by his loyal servants, he is greeted by Mal’Ganis, but seeing his plan foiled by Arthas, the dreadlord invites the prince to follow him to Northrend. As the city burns and Arthas heads to the freezing Northern lands, the prophet attempts to convince Jaina to head West once more. In Northrend, Arthas meets the dwarf Muradin who is an old friend of his. Muradin speaks about the runeblade called Frostmourne.
Arthas gets into more trouble. When he finds out that his father has recalled the expedition, he burns his own ships and blames the trolls. Then he goes looking for Frostmourne, and when they find it, Athas does not heed a warning from Muradin that tells him that the sword is cursed. Thinking that only by wielding Frostmourne can he lead his people to victory, Arthas pledges his bond with the sword which immediately starts to consume his soul to claim Arthas for the cause of the Lich King after he defeats Mal’Ganis.
Under the influence of his new weapon, Arthas does a lot of errands for his Lich King. After returning triumphantly to Lordaeron, he starts by killing his own father King Terenas. Then he is met by another dreadlord called Tichondrius, who reveals that the blade that Arthas is carrying was forged by the Lich King, and orders him to rally the Cult of the Damned for him to retreive the remains of Kel’Thuzad. He is then ordered to carry the remains in a special urn, which he later finds out to hold the ashes of his murdered father.
Meanwhile, in the Twisting Nether the dreadlords convene in their citadel to plot the Burning Legion’s invasion and total destruction of the human world. Six days later, Arthas is sent to attack the elven capital of Quel’Thalas. Sylvanas Windrunner comes out of the woods to stop his advance, but neither she nor the enchanted elfgates can stop him. Here he brings Kel’Thuzad to the sunwell in order to revive the lich once more. Together they go to a nearby orc camp in order to use their demon gate to receive further instructions from demon lord Archimonde. The demon lord tells them to seek the spellbook of Medivh in Dalaran so that they will be able to use it to summon him into their world.
The undead capture the mage’s capital of Dalaran, take the book and immediately start the summoning as the human armies try to stop them. After defending Kel’Thuzad to summon the evil Archimonde at the end of his damned campaign, Arthas realises rather pathetically what evil force he has brought to this world. A force that is both beyond his control and also rather evident that it will bring the destruction of the world that he has worked so hard to protect. Archimonde starts the destruction by crushing the city of Dalaran with his bare hands.
At the same time, that same demon curse starts losing its grip on the orcs which have been under its influence since crossing into this world through the dark portal. After the orcish Horde gets shipwrecked upon Kalimdor, Thrall starts to hear the call of the oracle and wants to complete his quest towards enlightenment. He also ends right in the middle of a struggle between the centaurs and the tauren. Thrall sides with the tauren and aids their leader Cairne Bloodhoof after he tells him that the oracle lies to the north beyond Mulgore. After rejoining his brother-in-arms, the bloodthirsty Grom Hellscream, he at first convinces Grom to accompany him in his quest to Stonetalon Peak and stay away from picking up fights with the humans.
Meanwhile as the Legion consumes Lordaeron, Tichondrius tells Mannaroth (who is the demon that the orcs made a pact with in the past) that the orcs have traveled west, and Grom tells Thrall that the humans, led by Jaina have also made the journey to the west. Not only does not he pick fights with humans, but when left alone Grom starts a battle with the night elves too, not to mention cutting all their precious trees. Cenarius, the protector of the forest is enraged and starts razing orcish buildings to the ground. While trying to defend himself against Cenarius' wrath, Hellscream finds out that by drinking from a nearby fountain he has become strong enough to fight back. Of course he did not know that the fountain was polluted by the diseased blood of Mannaroth, curse included, as the demon traveled west to catch up on them.
Thrall must cut his way through the humans in order to make it to the top of Stonetalon Peak, but this time it is Cairne that comes to his aid. Together they make it through the mountain’s cavernous passage-ways until they find the oracle, also finding Jaina already there. The oracle is none other than the prophet that has brought them all here. After revealing his errors of ages past, the prophet suggests that everyone join forces to combat a common foe. Aided by the tauren leader Cairne Bloodhoof, Thrall and Jaina work together to first remove Grom's curse and then counter back the undead attack. Grom still has pending business with Mannaroth however, and accompanied by Thrall, he sacrifices his life to defeat the demon once and for all.
Next, the orc and human armies go into Ashenvale forest to prepare for their last stand against the Burning Legion. The first reaction of the night elves is to throw everyone out of their forest. However the undead seem a much larger danger when they start to set up their base near the area where Furion Stormrage the druid lies sleeping in his barrow den. Guided by his lover Tyrande Whisperwind, the night elves get the horn of Cenarius and try to awaken Furion before their world is doomed.
The two elven heroes are not powerful enough without additional help, so they move further inside the forest to awaken the druids of the talon and underground to rouse the druids of the claw. However, deep within the Barrow Downs, Furion and Tyrande happen upon Illidan’s prison. Tyrande wants to free him, whilst Furion does not agree in spite of him being the prisoner’s brother and orders her to turn back. Thinking that Illidan will be another ally against the undead, Tyrande refuses to comply with the druid’s order and goes to free his brother. In order to prove himself, Illidan decides to hunt the demons that reside in Felwood forest. He finds Arthas waiting for him, who tells him that the only way for Illidan to free this land is by finding the skull of Gul’dan and destroying it. Illidan complies but instead of destroying the skull, he harnesses its power himself to become a demon too. Although he kills Tichondrius, he is banished from the woods by Furion for what he has become.
As battle rages on, the love bound night elves realise with the help of the prophet that the orcs and humans are also sacrificing themselves to protect their world. The prophet reveals that he is none other than Medivh, the last guardian, and he was responsible for bringing the orcs and the demons into this world in the first place. Medivh tells the leaders of the defenders that they can only overcome the undead by uniting their forces. Tyrande joins Thrall and Jaina Proudmoore to setup their defences against the undead, while giving enough time for Furion to setup up his trap at the top of Hyjal mountain.
As Archimonde advances towards the World Tree, the four heroes and their armies try to prevent him in reaching the top. However the undead army is vast, and each wave that is defeated is refreshed by an even greater wave of rotten attackers. Archimonde thinks that he has had the better over his foes. However as he starts climbing to the top of the tree, Furion blows his horn to unleash a terrible wisp attack. Archimonde is turned into ash, and Medivh has a nice story to tell his grandchildren.