We start in Gondor in 1940 of the Third Age. King Ondoher's daughter, Firiel (centre), is being courted by two men: Arvedui king[1] of Arnor2, and Earnil[3] Captain of Gondor (left). After some courtship, Firiel chooses Arvedui. Arvedui reforges the alliance between Arnor and Gondor, and he and the king each promise they will aid each other in time of need.
Cut forward four years, and Gondor is under siege by the Wainriders. Ondoher calls for aid from Arnor, but Arnor is itself under attack. Ondoher has to go into battle with only his own Northern Army of Gondor, though Captain Earnil is marching up from the south with the Southern Army. Ondoher fights a great battle on the Morranon - and loses, and is killed, along with his sons.[4] Earnil then arrives and leads the weaker Southern Army to a crushing victory in the Battle of the Camp, having previously destroyed the other army advancing from Harad.
With the throne of Gondor vacant, Arvedui tries to make a claim: he says that since his wife is by rights the heir, he should be called on to reunite the two kingdoms. Gondor, however, disagrees, and the Steward crowns the victorious general Earnil as king. Needless to say, this whole section is filled with interpersonal tension between our three leads. We can also make use of a couple of wizards here - say, Gandalf supporting Arvedui's cause, Saruman supporting Earnil's (because strength is the most important thing). Saruman, of course, is not yet evil - but that doesn't mean he's terribly nice.
[1] Actually his father is still king at this point, but since Araphant has no role to play in the story, it may be best to cut him. In the original, the marriage is a political one to seal an alliance between the two kingdoms - but romance is more fun.
[2] Actually Arthedain, the last of the three successor kingdoms to Arnor. I'm using 'Arnor' throughout for thematic reasons.
[3] Actually he has no such role in the story, but he's not specifically denied this spot. Besides, it makes the later conflict more personal.
[4] There's loads of stuff to work with here. Ondoher's second son was supposed to stay in Minas Anor as regent, but snuck out and joined the army of the proto-Rohirrim (ie, the Eotheod). He ended up getting killed in the Dead Marshes. If we establish him and his brother during the courtship stuff - Firiel is their sister, after all - we could do a lot with this.
Arnor is celebrating the birth[1] of Aranarth, son of Arvedui and Firiel - but even as the celebrations reach their height, word arrives that the forces of Angmar are on the move again, vaster than ever before seen. Arvedui has to set aside the celebrations and gather everything he can muster to face the Witch-King of Angmar.
He sends word to Gondor, but receives no reply. He calls on the elves of Lindon (under Cirdan), but Angmar manages to cut them off. The small army of Rivendell, being on the other side of the lines, makes it through, so Elrond is present[2] in Fornost for the siege. Also present are 500 hobbits of the Shire, who arrive unexpectedly and at the last moment[3] - and Gandalf.[4]
The forces of the Witch-King lock down around Fornost. The siege is long - about two years - but eventually Angmar breaks through the defences. Arvedui leaves his wife and son in Elrond's care and flees west and north, drawing the Witch-King's attention away from his heir. He takes the treasures of Arnor with him.
We have a fairly long semi-chase segment, with Arvedui briefly fighting on the North Downs, hiding in dwarven mines in the mountains, before ultimately being chased all the way up to the Ice-bay of Forochel. He hides with the Lossoth Ice-men until hope of rescue comes by way of a ship from Cirdan. The chief of the Ice-men begs him not to sail, but Arvedui ignores him - and the ship founders, and he drowns, with his two palantiri sinking around him.
[1] Actually his birth was decades before the Fall of Arthedain, but this is more interesting.
[2] There doesn't seem to be any information either way on this, but he's useful for plot purposes. Bear in mind Celebrian is still around at this point, so we could do something with her, Firiel, Arwen, and Aranarth. We should also send Glorfindel with them.
[3] This is needlessly dramatic, but fun.
[4] Whose role in this treatment is entirely made up. I guess he'd escape with Elrond later.
While the elves of Lindon are mourning the loss of Arvedui and the fall of Arnor, and preparing to face the advancing host of Angmar, suddenly a gigantic fleet from Gondor sails into the harbour. Earnur, son of the king, has led them to the aid of Arnor - but he's too late.
The armies of Gondor and Lindon ride out against the forces of the Witch-King and crush them utterly. Not a single orc survives. Along the way, they relieve the embattled host of Rivendell, and we get to see Glorfindel and Earnur fight the Witch-King, and drive him off - and we see Glorfindel utter the prophecy that 'not by the hand of man will he fall'. Earnur, humiliated by his defeat (the WK spooked his horse ^_^), refuses to listen.
With Arnor free but destroyed, Earnur returns to Gondor and his father. Very soon, though, messengers come from the north - from the Eotheod, and from Mirkwood (HI, LEGOLAS!): a black shadow passed through their lands, and they fear a new enemy has come, or an old one re-emerged.[1]
Suddenly, a massive host of orcs erupts from previously-empty Mordor, led by nine Black Riders - the leader of who is none other than the Witch-King of fallen Angmar. They burn their way through Ithilien, heading straight for Minas Ithil. King Earnil elects to meet them there, but in a horrifying battle he is defeated, and killed,[2] and Minas Ithil is taken. The city of the Moon is corrupted, and becomes Minas Morgul.
From his new stronghold, the Witch-King sends forth a mocking challenge to the newly-crowned Earnur, calling him a coward for failing in Arnor, and claiming his father died a coward's death - y'know, the usual stuff. Earnur wants to respond, but is persuaded against it by his steward - and then retreats to the tower where the palantir rests.
Some time later,[3] the WK repeats his challenge, and this time Earnur will not be dissuaded. He rides alone to Minas Morgal, passes through the gates - and we never see him again.
In Gondor, the people wait for a year for his return, with no luck. Eventually, after some debate, the royal council decide to appoint the Steward as ruler - but he insists on only taking the sceptre 'until the return of the king'.
The last lines in the film are the Steward - his name is Mardil - being asked 'What about the line of Isildur? What of the kings of Arnor?' He replies, 'That line died with Arvedui. There is no heir of Isildur now.' CUT to Rivendell, where we see Elrond smiling at the small boy Aranarth from the previous film. "Welcome, child. Welcome to Rivendell."
[1] These messengers are made up for the film, partly as an excuse to put Legolas in, and partly just for dramatic purposes.
[2] Holy Timeline Compression, Batman! In the book, the Nazgul return in 1980, they take Minas Ithil in 2002, and Earnil dies in 2043. However, since I can't find any reference to how he actually died, this seems good for film purposes.
[3] Seven years in canon, but I don't know what's best for the film. My image is of Earnur using the palantir in the meantime to mentally duel with the WK, and being weakened a la Denethor.
hS