Since I played this game back in 1982, I've yet to see another adventure game match the ambition and scope of this one. Sure, there have been longer games, but the sheer freedom to change course (and frustratingly, screw up the game forcing you to start over...) and interact with the game in the order that you would like is what sets this game apart. Basically, you start off with a Time Machine that can take you to any number of historical periods.
The catch of this game is that you can't take items back in time to a period where they don't exist or they will disappear forever. Thus, trying to determine the order in which puzzles must be unfolded becomes a challenge unto itself. You'll need items from many different times to complete some of the puzzles, so maintaining inventory and experimentation are the cruxes of the game.
If you ever get a chance to get your hands on this game, do it. You won't regret it.
