In Myst you arrive on the island of Myst and find two brothers trapped in prison books, Sirus and Achenar. They tell you of red and blue pages to find, scattered around five Ages, to free them. Later, you find a linking book to D'ni, where Atrus, their father, was trapped by them in that age. You also find instructions for how to access a missing page that goes into Atrus's now broken linking book back to Myst. You free him by finding the missing page and bringing it to him.
Riven starts where Myst ends off. You find Atrus still writing a book for the Age of Riven, which he is trying to edit to stabilize the Age long enough to rescue his wife, Catherine, and evacuate all the other people that live in Riven. You help him by going to Riven to rescue find Catherine and trap Gehn, Atrus's father who misuses the power to create books. After traveling through the five islands of Riven, as well as two other Ages, you trap Gehn in a prison book and rescue Catherine, who goes to evacuate Riven's civilians. The only way to signal Atrus to come to Riven is to open a hole to access the star fissure, a mysterious opening into the universe. You do this, but Riven begins to literally fall apart. Atrus and Catherine are reunited and link back to D'ni together. You, on the other hand, fall into the fissure, apparently now being able to go home.