This may be the only poem in the history of literature to begin with a disclaimer; Chesterton writes, in prose, an opening that explains the poem is not intended to be historically accurate.
In verse, he dedicates the poem to his wife, and reminisces about their travels together whilst researching locations he used in the poem.
The story begins with a description of the White Horse of the White Horse of Vale (there is a second white horse, on the North Yorkshire Moors, that is a slightly less crude rendition of the animal) The White Horse is a prehistoric hill figure of white chalk. The White Horse has been a witness to centuries of events and currently King Alfred has been driven into hiding by the invading Danish Vikings. Whilst he is in hiding he sees a vision of the Virgin Mary. She consoles him, and although she doesn't hint at his victory, she does talk of his salvation from harm.
Knowing that he has been visited by Mary, and feeling protected by the Gods, Alfred comes out of hiding in search of any Catholic chiefs and their followers who have not been driven from their churches by their invaders. He persuades a Saxon named Eldred to join him in his quest, which was no uncertain feet because the indigenous Wessex peoples and the Germanic Saxons were generally at odds with each other. He then summons the support of the leader of the Galls, and the Romans, telling them to gather their troops at the river, close to Egbert's Stone.
Alfred decides to disguise himself as a minstrel before he travels towards the river-hut, so that he can approach the Danish chieftains incognito. He carries a harp on his shoulder, and it is not long before he is captured by the Danes as he wanders near to their camp. They take him to Guthrum, their leader, who asks him to sing a song. Guthrum is fooled by Alfred's disguise, and does not realize that the peasant he has invited into his camp is Alfred of Wessex. He also fools the other Danes present, including three Earls, Harold, Ogier and Elf. Alfred sings songs about the history of Wessex, and then invites each of the Danes present to play the harp. Alfred then takes the harp back for himself and sings about his own perspective on the world. He leaves the camp in safety, the laughter of the Danes, who have greatly enjoyed his performance, ringing in his ears behind him.
Alfred sets off for the river-hut after leaving the Danish encampment and despite leaving for the rive hut after the other chieftains, he finds that they have not arrived yet, and he has arrived their first. He waits for them but whilst he does so an old woman offers him a cake, as she has been cooking, but in return she needs him to watch the fire for her for a while. Alfred agrees right away. He feels sorry for the woman because she has such a hard life. He allows his mind to wander to such an extent that he forgets to watch the fire; he burns the cakes and when the old woman comes back to the fire she is very angry, hitting him across the face with one of the burned cakes. His army arrive as she strikes him.
The Saxon army are not quiet, or stealthy, and as they ride through the woods many indigenous animals flee for their lives. This alerts Guthrum to the presence of the Saxon troops. Alfred is beginning to get nervous about the upoming battle. He ponders his own life and his character; he is basically a good man but has transgressed several times. He has been adulterous, and he has committed sacriledge. He is worried he will lose the battle because of his sins. He therefore asks the gathered soldiers to pray for his soul. Preparing to fight to the death, the chieftains declare how they each would like to be buried. As they meet the Danish army, Alfred and his chieftains are at the front of his army, and Danish earls are at the front of theirs, but Guthrum is riding at the back. Harold shoots an army at Colan, but Colan evades it, ahd throws his sword at Harold. He is an accurate shot and the sword hits Harold, killing him. Alfred praises Colan for his heroism, and gives him his own sword, leaving Alfred with the battle-axe he favors. The battle begins.
Eldred is a valliant soldier in battle, killing dozens of Danes, but his sword breaks unexpectedly and without it to defend himself is stabbed with seven spears. Elf reclaims his spear, because is is magic; it was bewitched when it was given to Elf by the water maidens in the English Channel, the body of water between Britain and France. The soldiers fighting under the leadership of Mark are frightened and begin falling back, but Mark rallies them, charging at Elf, and killing him with his sword. This gives the Christian troops more confidence in themselves and their ability to win the battle. They renew their efforts in attack. Mark repels Ogier easily; Ogier hides under his sword but Mark jumps on top of it, pinning Ogier underneath. Ogier manages to free himself slightly and stabs Mark, who falls off the shield as he dies. Ogier jumps up, throws away his shield, and tries to rally the Danish troops with a battle speech. They push the Christian army back, splitting it in two, with one half pushed back down one fork in the road, and the other half along the other fork in the road. Alfred and Colan are separated and Colan is killed.
At the White Horse Down, a small child makes piles of stones, piling one atop the other until they fall, and he begins again. The poet compares this action of the child to the action of Alfred battling the Danes - effectively two steps forwards, one step back. Alfred, back at the battle-ground, gives another rousing speech hoping to rebuild his men's confidence. Again his speech reinvigorates the troops and despite being beaten down by the battle they charge once again. As his army is about to be defeated, the Virgin Mary appears to Alfred again, and just as he was newly resolved after she first appeared to him, he finds that he is resolved again. He is separated from his men and finds himself surrounded by Danes. He recognizes Ogier. Ogier throws his spear at Alfred but the spear gets lodged in a tree, whilst Alfred swings his axe down onto Ogier's head and kills him. Alfred stands over Ogier's body and blows the battle sound with his horn.
This terrifies the Danes and they start to fall back again. The portion of the army that was separated from Alfred rejoins him and they fight behind Alfred, as eager as he is for victory. The Danes start to fall back but as they do so Guthrum experiences an epiphany and converts to Alfred's faith. After the battle he is baptized.
The end of the battle ushers in a period of peace in Wessex. Alfred is a man of culture and encourages education and learning. He sends explorers to other lands rather than armies, and he is humble. The Saxon people have great respect for the White Horse and take great care of it, making sure that is is weed-free and always visible. However, after a long period of peace, there is a change of leadership amongst the Danes and their army invades again. Alfred gets ready to fight again and readies his troops once more. Alfred confides in them that he doesn't fear the uneducated pagan invaders but he is scared about the educated ones because their battle plans have more strategy, and they are also persuasive enough to convert the Saxons to paganism. He is concerned that they will take over the lands.
Alfred leads his army to London, and they confront the Danes in attack for a second time.
