The plot is familiar but well executed. As future king of England, you as young Arthur must prove your worth for the magical sword Excalibur. There are many innovative puzzles that involve transforming yourself into various animals, and guidance from Merlin is always close at hand.
One of the best things about the game is the incredible atmosphere it creates-- Bates went through great pains (or joy, as he definitely enjoyed making this game) to make you feel as if you are truly in Arthurian England: the "Ole English" writing is top notch, and characters well-defined and believable. With dozens of location, a good mix of traditional puzzles and "timed" ones (i.e. you must solve it in a number of moves), and strong Arthurian atmosphere that reflects a lot of research, Arthur is an underrated masterpiece. Be sure to download the beautiful "Book of Hours" which was originally included in the game box-- you'll need that for your adventure.