Admittedly this is all from Britain's perspective, but anyway, the book is called "A History of Britain".
So there's this very influential family named Godwine. Daddy Godwine and King Edward aren't exactly friends because of this whole ordeal when Daddy Godwine misled Edward's brother, Alfred, ripped out his eyes, mutilated him, and let him bleed to death. Edward can't sack Daddy Godwine without turning the upper class against him and losing what little power he's managed to scrounge since he or his brother were the rightful heirs to the throne (Not that this means shit to Denmark and Norway).
Daddy Godwine reproduces, and rather rapidly his spawn die off, leaving Harold II to inherit all their lands until he has an estate nearly as large as the King's. Harold II doesn't exactly befriend King Edward, but is very aware of the animosity Edward has toward the Godwines, up until Daddy Godwine's death when he probably had a stroke during a feast and eventually died comfortably in bed.
So Harold goes on a crusade(s) to gain Edward's trust, and more territory in King Edward's name. Namely Wales. Wales fought viciously and bitterly, until the King of Wales was beheaded by his own soldiers to make Harold's soldiers stop attacking the villagers.
High on life, Harold gathered his ships and soldiers to conquer more land. A massive storm sends him off course until he finds some respite on Normandy's shores ... but not for long, because Duke William II of Normandy smugly captures William's ships and likely planned to execute everyone ... except he manages to extract a vow from William, which in return William, his brigade, and ships get to return home.
No one really knows what the vow was. But after Harold returns home he begins to campaign for William to become the new King of England after Edward dies. King Edward, who had been wed to Papa Godwine's daughter, has produced no heirs and is getting old.
More stuff happens, and on his deathbed Edward names Harold Godwine II the new king.
William of Normandy takes this as a personal slight since he'd been expecting to become the new king. He sends an angry letter saying Harold owed him, and Harold replies saying that he did everything he could, but he could not argue with a dying king's wishes.
Harold II was aware of the likely attack from Normandy and had his best warriors and other soldiers prepared to be called. About 13-16,000 men, and 6,000 horses.
MEANWHILE Harold II had a bitchy brother named Tostig who was generally useless. He was an exile, had a habit of pillaging his own serfs, and hated that Harold's loyalty was with Edward and not with the Godwine family.
Around the time Harold was gathering his troops to defend against Normandy, Tostig recruited an army, and traveled to appeal to King Hardrada in Norway. Hardrada had a weak claim on England's throne that was before even Alfred the Great's reign. Hardrada was reputed to be a fierce warrior who had survived many wars. Tostig enlisted his help while Harold was distracted with Normandy, probably promising Hardrada the throne, and in return Tostig would recover his earldom.
So. Both King Harold II and Duke William II had their armies prepared, and put themselves in a stalemate for 40 days. They waited for the other to attack, neither really able to afford the battle they were waiting for.
Harold's armies were only under obligation to serve in battle for 40 days. Men started disbanding and returning to their homes.
Tostig and Hardrada began their attacks at Fulford, outside of York on September 20, winning a surrender by the 26th.
Harold received news of the attacks on September 19, and arrived in York/Jorvik on the 24th, having regathered his troops again along the way.
I'm not good at battle descriptions, but Hardrada and Tostig fought until the bloody end. Harold had lost half his army.
Of course while Harold was in York, Normandy moved forward and set pre-made walls for defense. Their aim was to take the roads of London while Harold was gone, but William didn't get that far for the time being.
Harold left London on October 12th without seeing if he had any support from the northern earls. He couldn't let the Normans roam freely on English soil, and give the Normans an advantage if they overtook London while Harold was replenishing his armies.
What followed was a very long, tedious battle that lasted six hours. English soldiers kept rushing forward with the intention of wearing the Normans down ... it worked initially, but once the English got excited things began to get disorganized. Eventually they whittled away their own troops.
This is how I understand it at least.
When King Harold II took an arrow to the eye, the Normans had decidedly won.
Here's my obligatory fandom inclusion:
Look at them, blood and deceit has never been so adorable.
Well, it's less cute when you realize what follows is William the Conqueror/William the Bastard's brutal, brutal reign.