Wart accompanies his foster brother, Kay, on a hunting trip. After accidentally distracting Kay, Wart goes to retrieve Kay's lost arrow and ends up in the home of Merlin, a powerful wizard. While enjoying tea with Merlin, Merlin speaks to Wart about the value of an education and appoints himself as Wart's tutor. Wart returns to Sir Ector's castle with Merlin and is sent to the kitchen as punishment for worrying Sir Ector. Sir Pellinore, a friend of Sir Ector's, arrives with news of a tournament, with the English crown as a prize. Sir Ector decides that Kay can be knighted in time to participate. Ector also appoints the Wart as Kay's future squire for the tournament, which is to be held in London.
The next day, Wart accompanies Merlin on his first lesson. Merlin turns them both into fish and, at first, everything goes smoothly. Wart is soon chased by a large pike and manages to outsmart the pike at first. Thankfully, Wart is saved by Merlin's owl Archimedes. The adventure teaches Wart the value of brain over brawn. When Wart is transformed back, he rushes to the kitchen, where Ector gives him 6 demerits; 3 for being late, and 3 after telling the true story of his lesson.
Later, Wart is being forced to clean an entire kitchen as a result of his punishment. Merlin enchants the dishes to wash themselves and sneaks Wart out for another lesson. This time, the lesson is about gravity and (in an unexpected twist) male-female relationships; for this lesson, Wart is transformed into a squirrel and meets up with a female squirrel, who wants him as a mate. As Wart attempts to escape the amorous squirrel, he is put in danger of being eaten by a wolf. Wart is saved thanks to the female squirrel, but he is then transformed back into a human boy. Though he tries to explain, the female squirrel is heartbroken.
Back at the castle, Sir Ector begins reprimanding Merlin for using what he believes to be black magic in the kitchen. Wart tries to defend Merlin, but Ector piles more demerits on the boy and finally punishes him by both revoking his chances of making the trip to London and by making an unseen boy groom, Hobbs, Kay's squire in Wart's place. Merlin sees Wart's disappointment and apologizes, but notes that Wart could still make something of himself through an education.
Merlin then takes time to explain different concepts about how the world works, but Wart becomes so confused that Archimedes takes over Wart's education. After finding out that Wart is illiterate, Archimedes teaches the Wart to read and write. During a break, Merlin takes time to demonstrate a model airplane. Though the demonstration doesn't work, Wart is impressed anyway and relates his dream of flying to Merlin. While Wart is talking, Merlin transforms him into a sparrow. Wart then goes out flying with Archimedes. The flight goes well at first, as the Wart proves to be a natural at flying but, during the flight, Wart is chased by a hawk and, in escaping, ends up falling through the chimney of Madam Mim, Merlin's enemy. Though Madam Mim attempts to show Wart that her magic, which is self-serving, is better than Merlin's, Wart disagrees and so Madam Mim transforms into a cat and attempts to eat the Wart. Merlin's student is saved by the intervention of Merlin and Wart later observes the Wizard's Duel between the two wizards. As a result, Wart learns the value of knowledge and wisdom, as Merlin won by outsmarting Mim.
At Christmas time, Wart is reinstated as Kay's squire out of necessity (as Hobbs had come down with mumps). Wart is pleased, as he feels it is his only opportunity to advance. Merlin is angered that Wart still prefers war games over education and so the wizard teleports himself (quite unintentionally) to 20th century Bermuda.
On New Year's Day, Wart accompanies Sir Ector and a newly knighted Kay to the tournament. Everything is going smoothly when Wart realizes that he forgot Kay's sword at the inn. In an attempt to find a replacement, Wart pulls a magic sword from a stone that was in an old churchyard. When the Wart brings the sword, it is immediately recognized as the legendary Sword in the Stone, which is only supposed to be pulled by the rightful king of England. All the knights at the tournament force Wart to repeat the process, but before he can do so, Kay, Sir Ector and a few other knights attempt to pull it out, under the reasoning that anyone can pull it a 2nd time (which is proven to be false).
Pelinore and a knight called Sir Bart stop the other knights and allow Wart to pull the sword. He does so successfully, and everyone bows to him as the new King of England. Sir Ector takes the opportunity to ask Wart for forgiveness, which Wart feels is unnecessary. The crowd asks for the boy's name, and Sir Ector almost calls the boy Wart before correcting himself and calling the boy Arthur for the first time.
Time passes and Wart, now crowned King Arthur, feels unprepared for the responsibility of ruling a country. With the assistance of Archimedes, he attempts to run away, but every exit is blocked by cheering crowds. Merlin then unexpectedly returns from Bermuda and is pleased to see that Wart is now King Arthur and relates how famous the boy will become in the future.
- He is the first Disney character to be voiced by three different people.
- Unlike the other kings in Descendants media, Arthur dislikes the evolving technology of Auradon, leaving Camelot Heights in the Middle Ages (400 AD-1500 AD) by relying heavily on magic.