Because you're here, one of two things can be assumed. A) You've already played the game, know the story, and are just curious to see if I got it right, or B) You haven't played the game and are vaguely curious as to what the big deal is about, but are willing to spoil it for yourself in the process. Well, here it is:

The intriguing storyline concerns Atrus, one of the few remaining D'ni ¹, his sons, Sirrus and Achenar, whom tried to wrest control of Myst and all of its 'ages' ², but have ended up trapped in two prison books, devices Atrus himself wrote and put in the Myst library to stop any unwelcome visitors from disrupting his ages. Atrus' wife, Catherine, is mentioned but does not play an active role in the game. Your basic goal is to a) figure out what happened, and b) put things as right as they can be put, by finding Atrus. You also get to visit what nifty ages Sirrus and Achenar didn't destroy.

You may now return to your reguarly scheduled entry: Myst