This book follows the broken Fellowship and their adventures as they split into three groups.  Frodo and Sam make their way toward Mordor and along the way they meet Gollum.  Merry and Pippin have been captured by orcs and excape to meet the Ents.  Aragorn, Legolas and Gimli give chase to the hobbits and wind up involved in a struggle with the Riders of Rohan against Saruman's forces.  This book sets the stage for The Return of the King.

'Strange are the ways of Men, Legolas!  Here they have one of the marvels of the Northern World, and what do they call of it?  Caves, they say!  Caves!  Holes to fly to in the time of war, to store fodder in!  My good Legolas, do you know that the caverns of Helm's Deep are vast and beautiful?  There would be an endless pilgrimage of Dwarves, merely to gaze at them, if such things were known to be.  Aye indeed, they would pay pure gold for a brief glance!'

'And I would give gold to be excused,' said Legolas; 'and double to be let out, if I strayed in!'

'You have not seen, so I forgive your jest,' said Gimli.  'But you speak like a fool.  Do you think those halls are fair, where your King dwells under the hill in Mirkwood, and Dwarves helped in their making long ago?  They are but hovels compared with the caverns I have seen here: immeasurable halls, filled with an everlasting music of water that tinkles into pools, as fair as Kheled-zârum in the starlight.

'And, Legolas, when the torches are kindled and men walk on the sandy floors under the echoing domes, ah! then, Legolas, gems and crystals and veins of precious ore glint in the polished walls; and the light glows through folded marbles, shell-like, translucent as the living hands of Queen Galadriel.  There are columns of white and saffron and dawn-rose, Legolas, fluted and twisted into dream-like forms; they spring up from many-coloured floors to meet the glistening pendants of the roof: wings, ropes, curtains fine as frozen clouds; spears, banners, pinnacles of suspended palaces!  Still lakes mirror them: a glimmering world looks up from dark pools covered with clear glass; cities, such as the mind of Durin could scarce have imagined in his sleep, stretch on through avenues of pillared courts, on into the dark recesses where no light can come.  And plink! a silver drop falls, and the round wrinkles in the glass make all the towers bend and waver like weeds and corals in a grotto of the sea.  Then evening comes: they fade and twinkle out; the torches pass on into another chamber and another dream.  There is chamber after chamber, Legolas; hall opening out of hall, dome after dome, stair beyond stair; and still the winding paths lead on into the mountains' heart.  Caves!  The Caverns of Helm's Deep!  Happy was the chance that drove me there!  It makes me weep to leave them.'

'Then I will wish you this fortune for your comfort, Gimli,' said the Elf, 'that you may come safe from war and return to see them again.  But do not tell all your kindred!  There seems little left for them to do, from your account,  Maybe the men of this land are wise to say little: one family of busy dwarves with hammer and chisel might mar more than they made.'

'No, you do not understand,' said Gimli.  'No dwarf could be unmoved by such loveliness.  None of Durin's race would mine those caves for stones or ore, not if diamonds and gold could be got there.  Do you cut down groves of blossoming trees in the springtime for firewood?  We would tend these glades of flowering stone, not quarry them.  With cautious skill, tap by tap - a small chip of rock and no more, perhaps, in a while anxious day - so we could work, and as the years went by, we should open up new ways, and display far chambers that are still dark, glimpsed only by a void beyond fissures in the rock.  And lights, Legolas!  We should make lights, such lamps as one shone in Khazad-dûm; and when we wished we would drive away the night that has lain there since the hills were made; and when we desired rest, we would let the night return.'

'You move me, Gimli,' said Legolas.  'I have never heard you speak like this before.  Almost you make me regret that I have not seen these caves.  Come!  Let us make a bargain - if we both return safe out of the perils that await us, we will journey for a while together.  You shall visit Fanghorn with me, and then I will come with you to see Helm's Deep.'

'That would not be the way of return that I would choose,' said Gimli.  'But I will endure Fanghorn, if I have your promise to come back to the caves and share their wonder with me.'

'You have my promise,' said Legolas.  'But alas!  Bow we must leave behind both cave and wood for a while.  See!  We are coming to the end of the trees.  How far is it to Isengard, Gandalf?'

'About fifteen leagues, as the crows of Saruman make it,' said Gandalf: 'five from the mouth of Deeping Coomb to the Fords; and ten more from there to the gates of Isengard.  But we shall not ride all the way this night.'

'And when we come there, what shall we see?' asked Gimli.  'You may know, but I cannot guess.'

'I do not know for certain,' answered the wizard.  'I was there at nightfall yesterday, but much may have happened since.  Yet I think that you will not say that the journey was in vain - not though the Glittering Caves of Aglarond be left behind.'