In the game, creatures called "garrintrots" have imprisoned the player in Mt. Drash, and the player's task is to escape the dungeons.
The game itself is a very simple series of three-dimensional randomly generated dungeons, and the idea is to destroy all monsters that stand in way and exit to the next level. There is a time limit as well. The game doesn't employ custom graphics, but rather uses the VIC-20's standard set of graphical characters to draw the game scene.
The game was also notable for its soundtrack; using the 3-voice sound chip in the VIC-20 to provide background music. Very few games written for the VIC-20 featured such a background soundtrack that would become commonplace in games for the Commodore 64, Nintendo NES, etc.
In recent years, the game has been extremely sought after by collectors. First copies of the game were discovered and announced in 2000.
Source: Wikipedia, "http://en.wikipedia.org/wiki/Ultima:_Escape_from_Mt._Drash," available under the CC-BY-SA License.