It inspired me to dust off the CD I have of the game and play again! The good thing is I have forgotten most of the twelve levels, so this is like playing the game anew. Thief is definately one of my favourite PC games.
The basic plot is you are the character Garrett, a master thief and something of an expert in concealing himself when he doesn't wish to be noticed. His world is a curious mixture of medieval lords and soldiers, whom possess some modern technology, as well as zombies, ghosts and the religious order of the Hammers, on their guard against an evil figure called the Trickster.
Being a thief, Garrett has no friends and lots of enemies all too willing to kill him if they spot him. The hard setting makes the levels not only more difficult and complex but also doesn't allow him to kill anyone human (or else the mission for that level is failed); so this isn't a first person "shooter" game like Doom. In fact, he is only armed with a blackjack, sword and arrows. It is the Daddy of the sneak and peek, stealth, adventure game.
It is an exciting game which simply has to be played in the dark, wearing headphones. Sure, that can leave one with a nervous twitch under one's eye - you have to take extreme care not to give away your presence to the guards, and the sounds and snippets of music add to the intense, creepy atmosphere. "Return to the Haunted Cathedral" is more or less regarded by Thief fans as the scariest level of the entire game and really not for the faint hearted.