The adventure takes place on a crashed spaceship in the Barrier Peaks of the World of Greyhawk campaign setting.
The official description from the module:
It is the adventurers' job to find the source of these monsters and, if possible, put a stop to them.
Many monsters made their first appearances in Expedition to the Barrier Peaks, including the aurumvorax, froghemoth, and vegepygmy. Additionally, this module features robots and mighty powered weapons such as blaster rifles and laser pistols. This module also contains a fabulous suit of powered armor that allows characters to achieve incredible feats (characters have an 18(00) strength while wearing it). Many a Dungeon Master was dismayed to find players with characters wielding these genre-crossing arms, with some of the more conservative DMs (in terms of gameplay) banning them outright outside the confines of the module. Because of these special entities, the module is liberally illustrated to help aid players in understanding their mysterious encounters.
The players will discover that this "dungeon" consists of part of a downed spacecraft, supposedly its inhabitants having succumbed to some sort of disease and died off. Many of the ship's robots are still functioning, however, and the players must either avoid or defeat them (a few are benign, however, and may be ignored). Wandering police robots and an overzealous weightlifting instructor are just two of the automatons that must be dealt with. One of the module's most curious encounters is a medical robot trying in vain to find a cure for the disease which has already killed off the ship's crew.
Expedition to the Barrier Peaks was ranked the 5th greatest Dungeons & Dragons adventure of all time by Dungeon magazine in 2004, on the 30th anniversary of the Dungeons & Dragons game.