The Song of Roland's account of the Battle of Roncesvalles is not supported by history. According to Einhard's Vita Karoli Magni from the late eighth century, the attackers were Basques seeking revenge against Charlemagne's army for the looting of Pamplona. The following is the depiction in the poem itself, not a historical account.
Charlemagne's army is fighting the Arab Muslims in Spain. They have campaigned for seven years, and the last city standing is Saragossa, held by King Marsile, who is pictured not as a Muslim, but a follower of Mahumet and Apollin. Threatened by the might of Charlemagne's Franks, Marsile seeks advice from his wise man, Blancandrin, who counsels him to conciliate the Emperor, offering to surrender and giving hostages. Accordingly, Marsile sends out messengers to Charlemagne, promising treasure and Marsile's conversion to Christianity if the Franks will go back to Francia.
Charlemagne and his men, tired of fighting, accept his peace offer and select a messenger to Marsile's court. The protagonist Roland, Charlemagne's nephew, nominates his stepfather Ganelon as messenger. Ganelon, who fears being murdered by the enemy and accuses Roland of intending this, takes revenge by informing the Saracens of a way to ambush the rear guard of Charlemagne's army, led by Roland, as the Franks re-enter Francia through the mountain passes.
As Ganelon predicted, Roland leads the rear guard, with the wise and moderate Oliver and the fierce Archbishop Turpin. The Muslims ambush them at Roncesvalles and the Christians are overwhelmed. Oliver pleads with Roland to blow his horn to call for help, but Roland tells him that blowing his horn in the middle of the battle would be an act of cowardice. If Roland continues to refuse, Oliver will not let Roland see his sister again whom Roland loves the most. However, Archbishop Turpin intervenes and tells them that the battle will be fatal for all of them and so instructs Roland to blow his horn oliphant (an elephant tusk hunting horn) to call for help from the Frankish army. The emperor hears the call en route to Francia. Charlemagne and his noblemen gallop back even though Count Ganelon tries to trick them.
Roland's Franks fight well, but are outnumbered, until almost all his men are dead and he knows that Charlemagne's army can no longer save them. Despite this, he blows his olifant to summon revenge, blowing so hard that his temples start to bleed. After a few more fights, Roland succumbs to his wounds and dies a martyr's death. Angels lift his soul to Paradise.
When Charlemagne and his men reach the battlefield, they find the slaughtered bodies of Roland and his men. They pursue the Muslims into the river Ebro, where the Muslims drown. Meanwhile, Baligant, the powerful emir of Babylon, has arrived in Spain to help Marsile. His army encounters that of Charlemagne at Roncesvalles, where the Christians are burying and mourning their dead. The Franks fight valiantly. When Charlemagne kills Baligant, the Muslim army scatters and flees, leaving the Franks to conquer Saragossa. With Marsile's wife Bramimonde, Queen of Saragossa, Charlemagne and his men ride back to Aix, their capital in Francia.
The Franks discover Ganelon's betrayal and keep him in chains until his trial, where Ganelon argues that his action was legitimate revenge, not treason. While the council of barons assembled to decide the traitor's fate is initially swayed by this claim, partially out of fear of Ganelon's friend Pinabel who threatens to fight anyone who judges Ganelon guilty, one man, Thierry, argues that because Roland was serving Charlemagne when Ganelon delivered his revenge on him, Ganelon's action constitutes a betrayal.
Pinabel challenges Thierry to trial by combat. By divine intervention, Thierry kills Pinabel. By this the Franks are convinced of Ganelon's treason. Thus, he is torn apart by having four galloping horses tied one to each arm and leg and thirty of his relatives are hanged. Bramimonde converts to Christianity, her name changing to Juliana. While sleeping, Charlemagne is told by Gabriel to ride to help King Vivien and bemoans his life.
