Ultima V is probably the first Ultima that features a deeper story line and more fleshed out NPC's then any of it's predecessors. Still, the main storyline is the stuff pulp fiction cliches are made of. As a returning hero, you need to find the original ruler of Britannia who has gone missing and remove the evil tyrant that replaced him.
Still, Ultima V offers a lot of interesting features, like the 7 virtues that must be followed in order to gain the respect of the people, as well as a splendid magic system and interesting side quests that can be undertaken.
After Lord British has disappeared, his legacy has been continued by an evil character: Lord Blackthorn. Somehow under the influence of three so-called Shadowlords he has corrupted the very virtue system itself that made Britannia a go-happy place, where evil was almost redundant. During his quest the player (who is the avatar, the main player in every Ultima game) will interact with a host of colorful and interesting characters. As the avatar restores the shrines of the virtues to their former glory, he will uncover a storyline¬† which is deeper and more meaningful then any other Ultima game before.