Meet Me at Midnight (http://americanart.si.edu/exhibitions/online/midnight/default_flash.html)

Prepare to solve many puzzles. As you travel around the museum, a mysterious guide appears from time to time to offer help. The game is presented in a linear format, like a graphic novel come to life. Each segment takes quite a while to play. Once you win a code word, you can quit and return to that point in the game later.

If you want to skip the long exposition sequences, cut to the chase by clicking on the "Parents and Educators Guide" and then choose the "Links to Learning Elements" menu item. It gives you an overall index so you can jump around to the games, skipping all the walking around -- if you choose.