Ultima 5 uses the basics of the Ultima 4 engine, an overhead perspective of Britannia or one of many towns or dungeons, which use a rosette-compass 3D view. Many actions are expanded, including much more world interactivity and much deeper characters than the 'signpost' NPCs of Ultima 4.
In Ultima 5, you, the Avatar and Hero of Britannia, are called back to deal with a new threat - the tyrant Blackthorn, who, after the disappearance of Lord British, now rules the land by enforcing the virtues upon the will of the people, corrupting their meaning in the process. Behind Backthorn are the Shadowlords, anti-thesis to the three Principles. Their meaning and origin are explored later in the game, rounding out an immersive world and very meaningful plot.