The High King opens with Taran returning to Caer Dallben after resolving in Taran Wanderer that his life lay not in weaving, smithing, or pottery but with the enchanter Dallben. He arrives to a gala welcoming that is quickly cut short by the report that Arawn lured Prince Gwydion to the loss of the sword Dyrnwyn by assuming the shape of Taran in danger. Gwydion, determined to recover the sword by a quick assault on Arawn’s fortress, Annuvin, sets off with the companions (including Eilonwy, back from learning to be a princess, Prince Rhun, and the former giant Glew [shrunk by one of Dallben’s potions]) for King Smoit’s castle.  The latter group, including Fflewddur, become separated from Taran and Gwydion, who, arriving first at King Smoit’s castle are captured by surprise when Magg turns out to be the master of the castle. Alerted by Eilonwy’s suspicions, Fflewddur’s reconaissance discovers the state of the castle and with the help of the departing Fair Folk waypost guardian Gwystyl embark on a plan to regain the castle. Although the plan is initially discovered, Prince Rhun routs the defenders by faking a large army. Sadly, his rare success is fatal.

With the discovery that Arawn’s men are in posession of King Smoit’s domain and the news that Arawn is amassing an army to attack the main defence of Prydain, Caer Dathyl, Gwydion abandons pursuit of Dyrnwyn and begins gathering his liegemen to defend Prydain. Taran aids by obtaining the assistance of the men of the Free Commots but bitter mourns his failure to protect his valued friend and mentor Annlaw Claw-Shaper from the marauders that attack his growing army. The final reinforcements, King Pryderi, whose host is larger than the combined defence, arrive but Pryderi announces his betrayal, his decision that Prydain is better served by a false ally with Arawn than in a useless defence against him. Realizing that only with Pryderi’s help was there any chance of standing against Arawn, mournfully do battle against their former ally. But the battle ceases early, when Arawn’s legions of deathless Cauldron-Born arrive and sack Caer Dathyl, now defended only by the elderly High King Math. With the sack of Caer Dathyl, effective resistance to Arawn is ended and the last remaining treasure house of Prydain, the hall of the Bards, which houses the lore and history of the Bards is destroyed. The Cauldron-Born, who cannot stray far or long from Annuvin depart the next day.

The only glimmer of hope now lies in pyhrric assault on Annuvin by Gwydion with the ancestral ships that the Sons of Don used to sail from the Summer Country to aid Prydain. Since only Gwydion (and the deceased King Math) know the location of the ships, Taran’s army is given the task of delaying the Cauldron-Born’s return to the now undefended Annuvin. Quickly marching to the retreating Cauldron-Born army, Taran’s army successfully defends the Red Fallows, once a land of overflowing plenty, now long destroyed by endless wars and the quick and easy way to Annuvin. The victory is with great cost to Taran—his long-time teacher Coll dies defending a breach, never to return to his beloved farm at Caer Dallben. A second attack on the Cauldron-Born in the hills ends with the defeat of Taran’s army, allowing access to the Red Fallows, and the loss of Eilonwy and Gurgi, who are found to be missing.

Doli, sent by the Fair Folk King Eiddilig to aid in the defence of Prydain, leads the army to a shortcut through an abandoned Fair Folk mine but the selfish greed of Glew causes the mine to cave in with his reckless search for the huge gems of the mine tailings. Eilonwy, rescued from her captor Dorath by wolves, arrives across the valley from Taran and briefly shines her bauble with the brightness of the sun to reveal to Taran the party of Hunstmen, the mortal officers of the Cauldron-Born army. Warned against following his fatal path and protected from attack by the wolves and bears that attack the Huntsmen, Taran’s army is saved from another defeat by Doli’s men melting a lake onto the Hunstmen in the valley below by burning bushes to heat it.

In the meantime, King Pryderi rides to Caer Dallben where his small band is quickly scattered by Dallben’s enchantments of wind, earthquake, and fire.  Assaulted by Pryderi, Dallben notes that he who sought to free Prydain from Arawn by his allegience to Arawn finds now himself less his own master as a servant of Arawn, betrayed by the unachievable task of killing Dallben and betrayed again by the useless risk of getting The Book of Three, since it cannot serve Arawn but does cost Pryderi his life.

Back in the mountains, Taran’s party is attacked by gwythaints, fierce eagle-like birds trained by Arawn, but is driven off by a band of crows.  Kaw, Taran’s crow, was sent to reconnoiter Annuvin but was almost killed by gwythaints and barely makes it to animal healer Medwyn’s valley. There Medwyn summons the wolves, bears, and crows to fight against Arawn, with the results previous discussed. The party rescues Achren, former (evil) ruler of Annuvin and Prydain, who had departed Caer Dallben when Dyrnwyn was stolen to wreck her revenge on Arawn. As a result they suffer a snowstorm that obliges Fflewddur to sadly burn his beloved harp (even though it breaks its strings when his exaggerates) as firewood to keep them from freezing to death.

Achren tells them of the hidden back-door to Annuvin, across Mount Dragon.  The party scales the heights, Taran falls, is rescued by the gwythaint that he rescued in The Book of Three, and guided by a trick of the whispering wind, finds Dyrnwyn hidden under a rock. Its true power is revealed as Taran kills an attacking Cauldron-Born with it and sees that with the one, all the Cauldron-Born die, forever to truly rest. Arawn is killed, quenching Dyrnwyn’s flame and Annuvin is finally destroyed.

With Arawn vanquished, the Sons of Don have fulfilled their mission and set sail for the Summer Country, where all is happiness and death never comes.  Gwydion, the last of the House of Don departs, as must Dallben, Fflewddur, and Eilonwy. Taran and Gurgi are offered a spot on the ship as well, but Taran refuses because men better than he are dead and cannot go with them and because much remains to be done in Prydain to restore it to the wealth it once had. Fflewddur offers a parting gift of his remaining harp string, Gurgi gives a small coffer he found in Arawn’s treasure house which discovers contains “the secrets of forging and tempering metals, of shaping and firing pottery, of planting and cultivating,” the knowledge that “Arawn stole long ago and kept from the race of men.” Dallben crowns Taran the new High King, gives Taran The Book of Three, the book that told the past, present, and the future (but the future only in veiled words). Eilonwy gives Taran her bauble, and then, chooses to remain with him.

The High King is less a book of themes than an explanation of Prydain.  Here we see Arawn’s designs: after the attack of the Horned King in The Book of Three fails he gathers more Cauldron-Born for an attack but cannot actually attack because Dyrnwyn will destroy his deathless army.  Once Dyrnwyn is safely hidden, the attack can be carried out safely.  We also see more clearly who Arawn is, the Lord of Death who seeks to bring a living death upon Men. His character, too becomes more clear—a inveterate betrayer, betraying Achren, betraying Magg, and lastly, betraying Pryderi. Even his Iron Crown is enchanted so that it kills Magg who tries to steal it as Arawn flees Drynwyn. The Sons of Don, then, journeyed on a noble mission, to leave their country and become mortal to serve the men of Prydain in destroying the evil that they cannot destroy. Taran, too, finds the answer to his parentage (his parents are of unknown rank, their baby being the only survivor of a battle) and finds his search to be a hero ended as High King.

Even the unique autumn-chill quality of Prydain is explained, although it has been hinted at in broad terms throughout the series. Prydain is an Eden after the Fall, a land of enchantments originally used, presumably by the House of Llyr to bring peace and joy to men. But evil enslaved the enchantments and the race of men and men had to be rescued by the Sons of Don. Now that the great evil is destroyed, “men shall unaided guide their own destiny” as Dallben explains and enchantments have no part in that.  Prydain is fundamentally an echo of our own life—sadness, joy, honor, courage, love, and especially sacrifice to help others. The witch Orddu says, as she crystalizes Taran’s resolve to give up everlasting happiness for the toil of helping the race of men, “the pattern [of the tapestry of your life] is of your choosing and always was.” And Taran succinctly states the theme of Prydain: “But here, here it is unfinished.”