During their confrontation on the slopes of Amon Hen, Boromir takes the Ring from Frodo. When Frodo tries to fight back, he is knocked unconscious. Boromir briefly considers rejoining the Fellowship, but ultimately returns to camp only wearing the Ring. He quietly collects his pack and flees to the south: down the North Stair, and along Anduin. He knows it is roughly 200 miles to Minas Tirith, even if he takes the direct route - which runs through the delta-like Mouths of the Entwash.
Back at Parth Galen, the Fellowship remain in silent contemplation until a group of orcs attack them. They fight them off, but realise that Frodo (and Boromir, who Sam observes has been missing for some time) may be in danger. Much as in the book, Sam, Merry, and Pippin race off; Legolas and Gimli follow in the general direction of Merry and Pippin, while Aragorn runs after Sam.
When Legolas and Gimli get caught up fighting orcs, Merry and Pippin are captured. Aragorn and Sam, meanwhile, come across the site of Frodo and Boromir's struggle - and discover that the Uruk-Hai got there first. Aragorn discovers that Frodo has been taken - and by the lack of blood, he is still alive.
The four remaining members of the Fellowship regroup at Parth Galen after some hours. Aragorn lays out the whole tale, and they realise there are two options open: to follow Boromir and try to reclaim the Ring, or to chase the orcs and try to rescue the Hobbits.
Ultimately, after some discussion, there is no choice. Aragorn will not let Boromir escape - Sam will not abandon Frodo. Aragorn decides that Legolas will accompany him, while Gimli assists Sam. The Fellowship is well and truly broken.
Frodo, Merry, and Pippin are halfway across Rohan. Pippin, as in the book, manages to free his hands and drop his brooch. Frodo, while conscious, is uncommunicative.
Sam and Gimli descend the East Wall of Rohan and set out onto the plains. While their descent is about as fast as in the book (Sam being motivated by concern for his master), the hobbit is unable to set the same pace as the Three Hunters from then on. Without Aragorn or Legolas to guide them, they are unable to quickly locate the trail, but ultimately (and after long discussion over the discovery of orc bodies from the quarrel) set out on a beeline for Fangorn. They walk through the night, unwilling to lose the time.
Boromir, his strength increased by the Ring (which he has yet to take off), makes good progress across the delta. He does, however, leave a clear trail - and is unable to move as fast as Aragorn and Legolas. The delta is treacherous at night, and both groups stop to camp, only a few leagues apart.
Eomer and his eored encircle Ugluk's orcs on the eaves of Fangorn.
Sam and Gimli continue to struggle across Rohan. Unknown to them, they have already lost the trail. Pippin's brooch lies where it fell until the world's end.
Aragorn and Legolas catch up with Boromir. Legolas is able to wound him with an arrow, but Boromir - invisible, and a strong swimmer - throws himself into a branch of the Entwash. Unlike Isildur long ago, he is not betrayed by the Ring, but makes it down to Anduin. He continues to swim downstream for some time - holding onto a convenient floating log for some of it - before returning to land to continue his journey.
Ugluk's orcs are killed. The three hobbits manage to escape much as in the book, with Merry and Pippin having to practically drag Frodo. They encounter Treebeard, and that night the Ent-draught is able to revive Frodo sufficiently for him to tell his friends what has happened.
Sam and Gimli are by now thoroughly lost, wandering in the vicinity of the Entwash. Gimli determines that the best course of action is to head upstream until they reach the ford at the Entwade - or the forest.
Boromir continues down the west bank of Anduin until he comes to the fortress-island of Cair Andros. Made cautious by Aragorn's 'betrayal', he removed the Ring for the first time and commands half of the garrison to accompany him to Minas Tirith. They comply willingly - partly because he is their commanding officer, and partly because of the dominion granted to Boromir by the One Ring.
Legolas and Aragorn continue on a beeline for Minas Tirith. They actually pass Cair Andros before Boromir reaches it, and continue on through the night.
February 30 (yes, in the LotR calender, that's a real date)
The Entmoot is convened in Fangorn. It is made more urgent by Treebeard's knowledge of the One Ring - Frodo was in no state to conceal the information.
Sam and Gimli reach the Entwade, where they are taken prisoner by the guards. They lack Aragorn's commanding presence, but Sam is able to at least help them keep their lives by his simplicity - though hotheaded Gimli is kept in chains. The pair are escorted south towards Edoras.
Aragorn and Legolas reach Minas Tirith early in the morning, but are unable to gain an audience with the suspicious Denethor. Aragorn debates declaring himself, but ultimately decides against it.
Late in the day, Denethor suddenly makes his move. Informed of both Aragorn's identity and Boromir's approach (and the Ring!) by the Palantir, Denethor mobilises his army and sends them across the Pelennor to escort Boromir home. Aragorn is taken prisoner, though Denethor insists he is an honoured guest ('the Chieftain of our fallen Northern kin'); Legolas barely manages to escape into the city.
As night falls, Boromir reaches the city and ascends to converse with his father. Denethor thanks him for bringing Gondor the Ring, and requests that he turn it over. Boromir hesitates - and then refuses, for only one hand can wield the Ring, and it has had days now to work its will on him. Denethor, enraged, orders the guards to seize Boromor. Boromir puts on the Ring, and uses the dominion it grants to control the guards. Unfortunately, with little practice at using the Ring, he forgets two crucial facts:
He still casts a shadow.
And Denethor has a knife.
On the stroke of midnight, Denethor stands in front of the Palantir, bearing the One Ring, and commands that Sauron humble himself before the strength of Gondor. He claims that he will allow the Dark Lord to live - if he surrenders all his armies, fortresses and weapons to Denethor. This is essentially the same ploy tried by Ar-Pharazon in the days of Numenor. Denethor knows this, and believes he can resist falling into Sauron's trap.
But Sauron has learnt, too. Despite Denethor's possession of the Ring, Sauron decides to attack. Now.
In the book, when Aragorn reveals himself to Sauron in the Palantir, Sauron panics. He thinks Aragorn might have the Ring, and launches a massive assault to stop him from using it against him. But that was on a suspicion. This time, Sauron knows exactly where the Ring is - and the character of the mortal who has it. He knows that if Denethor is given time to get used to the Ring, Sauron will fall - but, conversely, if Sauron can take the Ring, all of Middle-earth will be his.
The only option is an all-out assault. Everything. The armies that had been in Dol Guldur, preparing to assault Lorien, Mirkwood, Dale, and Erebor, are immediately mobilised to head south, through the Brown Lands, and then on through Ithilien. Orcs pour out of the Morannon, Minas Morgul is emptied, and even the Mannish armies from the South and East are hurried on their way. Within a day, everything south of Mirkwood and east of Anduin becomes a sea of Sauron's forces.
And through the Palantir, Sauron commands Saruman to move as well - immediately. Isengard is emptied; the Second Battle of the Fords of Isen takes place a day early as the Uruk-Hai march in full daylight. Not even stopping to consolidate, Saruman's army marches east along the Great West Road.
Sam and Gimli have been brought to Edoras, where they find Theoden still under Wormtongue's spell. Grima, knowing of Saruman's fascination with halflings, speaks of 'fairytales out of the north', and manages to convince Theoden not even to grant an audience to the travellers. They are commanded to leave Rohan by the fastest route. Grima intends for them to head west, towards the attack he knows is coming (though he is expecting it a day later); instead, Gimli decides to scale the White Mountains. The travellers are escorted south to Dunharrow, and begin the ascent.
In Fangorn, the accelerated Entmoot is drawing to a close when a figure in white appears. To the shock and delight of the hobbits, it is Gandalf! He already knows the Ring has been taken - he has had to remove Narya to keep Denethor from feeling his thoughts - and he fears that Sauron is already moving.
With Theoden still under Wormtongue's influence, there is no Muster of Rohan. Saruman's army slaughters its way across the Westfold, capturing the barely-defended Hornburg, and continues on towards Edoras.
All is not well in Isengard, though. Overnight, the Ents have marched on the Wizard's Vale and laid siege. With Gandalf assisting them, they break the tower of Orthanc and bring Saruman out in chains. Seeing the three free halflings, Saruman spits and curses - but he is powerless to overcome Gandalf the White. Now Gandalf must make a choice - to attempt to defend Rohan, to ride hard to Gondor, or to take Saruman north to Lothlorien, imprisoning him for good and securing Galadriel's aid against Sauron.
Of course, Galadriel is already on the move - as is Thranduil, and even the armies of Erebor and Dale. Shocked by Sauron's sudden retreat, they have launched their own attacks on the running armies. Before night falls, Galadriel has torn down the walls of Dol Guldur. As Sauron's forces move south, they are hotly pursued.
After a day of planning Legolas manages to rescue Aragorn from his gilded cage. From the walls of Minas Tirith, they watch the shadow rise in the East - a great black arm stretching towards a city still thronging with civilians. They know that whichever side wins, there will be no true victory - not unless they can wrest the Ring from Denethor's grasp, cross a hundred miles of hostile terrain, and somehow - despite everything - destroy the One Ring.
And Denethor? Denethor waits, with his Ring and his Palantir. He is watching everything. And he is planning.
So much is moderately predictable. Beyond this point, we're into the realm of storytelling - not prediction.
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