In Medieval England, the death of the King creates a power vacuum, leaving the kingdom in turmoil as no one can agree on the rightful heir to the throne. As the threat of civil war looms large, a remarkable event takes place in London: a stone and anvil miraculously descend from the heavens, with a sword firmly embedded within. Inscribed on the hilt are the words, "Whoso pulleth out this sword of this stone and anvil is rightwise king, born of England." Despite the numerous attempts made by various knights to pull the sword from its resting place, none are successful, and eventually, the sword is forgotten as the country plunges into the dismal darkness of the Dark Ages.
Amidst this chaos, Sir Kay and his foster brother, Arthur, affectionately known as 'Wart', venture into the woods for a hunting excursion. When Arthur mistakenly causes Kay to miss his shot at a deer, he promises to retrieve the lost arrow. This quest leads him to a small cottage inhabited by none other than Merlin, a wizard of great repute, and his wise pet owl, Archimedes. With a cup of tea shared, Merlin expresses his anticipation for Arthur's arrival and decides to accompany him back to his castle. After gathering his belongings, Merlin and Arthur embark on an enchanting journey.
Upon their arrival at the castle of Sir Ector, Arthur is thrust into the kitchen, immediately taking on chores. Merlin introduces himself and Archimedes to Ector, showcasing his magical prowess to the astounded lord. Ector, initially skeptical, ultimately permits Merlin to stay, housing him in the crumbling Northwest Tower of the castle.
Not long after, the arrival of Sir Pelinore with news of an upcoming jousting tournament grabs everyone’s attention. This tournament promises that the victor will ascend to the throne as the new King of England. Excited by the prospect, Ector urges Kay to train diligently, even promising Arthur that if he proves himself useful, he can become Kay's squire.
However, Merlin has grander plans for Arthur. Aware of Arthur's true potential and destiny, Merlin aims to guide him toward greatness. During a stroll by the castle's moat, Merlin reveals his ability to transform into a fish, exciting Arthur. A thrilling underwater escapade ensues as both become fish to explore the waters. Yet, danger lurks in the form of a giant fish that chases after young Arthur, but he is saved by Archimedes.
Once back in their human forms, Arthur’s neglect in helping Kay results in him being punished with dishwashing duties. In a twist of fate, Merlin enchants the kitchen utensils, resulting in a chaotic, spellbound kitchen scenario that requires intervention. After breaking the spell, Arthur finds himself being unjustly blamed, and Ector begrudgingly punishes him, which leaves Arthur heartbroken.
Determined to help Arthur overcome this adversity, Merlin wishes to educate him further. Yet, as the lessons progress, Archimedes expresses skepticism about Merlin’s teachings, pondering the relevance of the knowledge in their bleak Dark Ages. Frustrated, Merlin ultimately hands over the teachings to Archimedes while he experiments with a model plane, attempting to demonstrate a fantastical future where humans can fly. Despite its failure, Arthur shares his dream of soaring through the skies, prompting Merlin to use his magic to make that dream a reality. Their flying lessons are cut short when a hawk chases Arthur into the woods.
Arthur soon finds himself entangled in the mischief of Madam Mim, a wicked witch who showcases her malevolent magical abilities. But just as she plots to unleash her powers against Arthur, Merlin intervenes, spawning a riveting Wizard's Duel between the two. The duel unfolds with a series of transformations until Merlin cleverly shifts into a germ, causing Mim to fall ill. With the witch neutralized, Merlin, Arthur, and Archimedes make their way back to the castle.
As Christmas approaches, the castle buzzes with excitement around the tournament. However, chaos ensues when Kay’s squire, Hobbs, falls ill, forcing Ector to hastily choose Arthur as a replacement. Despite his delight, Merlin feels torn—hoping for greater things for Arthur, he expresses his frustration before venturing off in a huff, exclaiming, "Blow me to Bermuda!"
In London, the tournament is ablaze with anticipation. In the midst of the impending battle, Arthur realizes he forgot Kay's sword back at the inn. Racing against time and facing locked doors, he stumbles upon the sword in the stone, effortlessly pulling it free. Unbeknownst to him, this act sets the stage for a monumental revelation. When he hastily rushes back to the tournament, Kay and Ector view the sword with disbelief; the inscription proclaims Arthur's right to wield it before a crowd of knights and peasants.
Despite the chaos that follows when the sword becomes immovable again, Arthur’s determination leads him to pull the sword from the stone a second time. Ector kneels in humility, pleading for Arthur’s forgiveness for his previous actions. With the legend manifesting, Arthur is crowned the King of England. Yet, in the whirlwinds of change, Arthur finds himself bewildered, calling upon Merlin for guidance. With a triumphant return from his unexpected journey to Bermuda, Merlin reassures Arthur, cherishing the fulfillment of the boy’s destiny and the promise of legends yet to be told, forever embracing the mantle of "the once and future king."
