Lyanna Mormont dies in service of her King, or rather would have if not for her adviser who forced her away from Winterfell as it started to be overrun. So she ends up in Torrhen's square. Winterfell has fallen. [ch title : Don't You Were a Child ? {bad grammar is on purpose ;)}]
Meanwhile, the Lannister army and the golden company, now led by Arya as Cersei, reach Moat Cailin where they will try to stop the dead's advance. They make a good stand, but end up being overrun after about 10 hours with the arrival of the ice spiders and have to abandon the castle and retreat. Only the mounted troops stand a chance as the ice spiders just roll over everybody else.
The iron fleet makes it to Torrhen's Square and picks up the survivors right in the nick of time as the siege had started and it was already looking bad. A few of the ships take the Saltspear upstream to the Fever river, ending up 35km away from Moat Cailin where they try to rescue the Lannister army. Which they manage to do with Bran's help thanks to his newfound power to slow down the dead and spiders.
The fleet then sails to Seagard where the army then walks to the Twins. They figure it's their last chance to stop the whole white walker army before it splits up and it becomes a nightmare to handle. Before they've lost basically.
[Sansa will take Sandor aside one day and apologize for not leaving King's Landing with him when he offered. She was scared. Partly of him. But the sight of him after all these years awoke something she thought Ramsay had broken. They end up having sex (though he didn't want to at first. Him : "Stop it. What's gotten into you girl ?" Her : "I am the lady of Winterfell now ser Clegane. I advise you not to forget it."). {impending death and all that for those who say it's out of character. And it's fanfiction. Embrace smut !}] [ch title : Tonight's the Knight's]]
The remaining human army baits the white walkers into trying to cross at the Twins in order to buy enough time for all the wildfire from King's Landing to be brought to them so they can end them. That is the job of a small part of the iron fleet which was left in King's Landing. It ferries the wildfire and gets ready to use load their catapults with it. Their targets will be the white walkers. Once these are gone all the living will have to do is mop up the remaining dead with no one left to create more of them.
Before the fighting starts however, there's a negotiation with the Night King. The fighting stops but all know that if they kill him, chaos will resume.
The rest of the white walkers stand not far from the negotiating table, as deterrents. That's what the humans were waiting for. The ships fire their catapults and rain wildfire behind them, isolating them from the main army (though they still have about a thousand dead with them in the circle of fire). There are weak spots in the circle as well so Bran has to freeze the dead. [ch title : Do Your Worth]
[Final showdown] : The tent where the negotiations were happening gets ripped to shreds as the Night King throws the table away with immense strength and arrows rain down. Jon doesn't miss a step and flies down with Rhaegal, covering the retreat of the negotiators. He immediately jumped down off the dragon to challenge the King while Rhaegal turned to roast the advancing dead who are close to just swarm them and effectively take the heads of the human army.
After a quick fight, the Night King is able to incapacitate Jon by twisting his arm and forcing his own sword through his leg.
The Night King now advances towards Bran (sees the future and can freeze a certain amount of undead in place, so annoying and clear target) who's in Sansa's arms after getting pushed back by the table. Arya places herself in front of them in a defending position before he can reach them though (she's not using a face). Then Brienne and Sandor do the same for her and position as a shield to Arya (which is a big deal since Brienne and Arya had a falling out when Arya admitted to killing Jaime), and are shortly thereafter joined by Tormund and Jorah. The fight starts, it's a 5 vs 1.
Arya runs to Bran and tells him to release his hold on the dead he was keeping at bay (and so endanger many people) and focus on slowing the Night King down. Nothing else. And she runs in to help the fight.
After a few minutes, it becomes a 2 vs 1 when only Arya and Brienne remain. Jorah is dead. Sandor is incapacitated but still had the strength to push the magic purple sword to Arya as she was about to be killed but is able to deflect the blow thanks to the sword. From then on, thanks to their sparring for months, Arya and Brienne become an incredible team, swapping weapons, …
Tormund, wounded, manages to get back up and rush the Night King. He tears off the armor piece protecting the King's right forearm but gets killed in the process. However, that makes Brienne able to cut the arm off.
The fight continues, and although not as one sided as it used to be, the Night King still seems to be about to come out on top. Arya has lost an eye during the skirmish and is out of the fight, leaving Brienne as humanity's last hope. But as she's forced to a kneeling position, Arya jumps on the back of the Night King and stabs him in the neck with her Valeryan dagger, cutting the thing [technically accurate term ^^] holding his breastplate in place in the process. She hangs on to her dagger and uses her other arm to expose his torso which is Brienne's opportunity which she seizes immediately and pierces him through and through with the magic purple sword. It does the job and the Night King is finally brought down.
But unfortunately it also goes through Arya's arm, the one which was holding the dagger in the Night King's back. It will have to be amputated.
Her arm situation stabilized (meaning she won't die from an infection), Arya goes to meet her white walker friend and they'll accept their fate in the end, that as long as there's a single white walker alive, humanity will never be at peace.
Them : "Don't you forget me Arya Stark."
Arya hugs them and after a few seconds stabs them. She only lets one tear out, which is the first in years, and walks away. [Chapter title : Youth Talking Blues] {sidenote : I'm so bummed I won't write this relationship because damn it would have been great, both heartwarming and heartbreaking}
In the end, Jon takes the last surviving dragon to King's Landing melts the Iron Throne.
Possible Jon lines : "How many people do you reckon died for that glorified stool ?"
"Soon there will be no more dragons."
"This throne has only brought misery. The kingdoms' shall rule themselves once again. As they always should have."
And he leaves with Rhaegal to cleanse Valyria then explore Sothoryos, leaving Sansa in charge of Winterfell. [ch title : Fils de Feu]
Here's a recap of the main characters' status at the end of this story :
Dead : Tormund, Edd, Gendry, Beric, Tyrion, Daeneris, Sandor, Davos, Varys, Podrick, Jorah, Samwell, Gilly, Theon, Cersei, Gregor, Jaime, Melisandre (yeah she was there but I had nothing for her)
Alive : Lyanna, Qyburn, Meera, Daario, Robin
-Jon departs for Sothoryos, seeking other dragons for he cannot let Rhaegal be the last of his kind and alone. But really it's an excuse for him to be alone, far from it all. He has PTSD.
-Sansa is Queen in the North and relieves Brienne from her oath.
-Brienne is knighted and offered to be master-at-arms of Winterfell (does she accept ? It's unknown)
-Bran is Sansa's advisor.
-Arianne Martell and Tyene Sand both rule as co-Queens of Dorne.
-Little Sam, orphaned, is left with either Edmure and Roslin or Missandei and Grey Worm. {you decide ! ^_^}
-Euron sails back to the Iron Islands.
-Arya sails to the west, trying to find land.
-Yara sails with her, getting away from this land she can't stomach anymore.