Taking place almost immediately after Myst, Riven sees you, the Stranger, returning to D’ni, presumably to ask Atrus whether there’s any way for you to return home. Atrus is still frantically scribbling away in his book and looks greatly relieved to see you. Hastily explaining that he needs your help to rescue his wife, Catherine, but that he cannot come with you, he hands you a small journal, to read at your leisure, and a prison book, much like the ones which contained Sirrus and Achenar. This, as he clarifies, is to be used to capture someone called Gehn. He also begs you to warn him once you find Catherine and proceeds to hold up the book he’s been writing in. And if his anxiety wasn’t already enough to unsettle you, the fact that the book’s linking panel is a flickering blur is a sure-fire sign that something’s not quite right with this age you’re going to.
Your arrival isn’t a smooth one and provides you with an immediate goal which supersedes the long-term objective of finding Gehn and Catherine. You can then glean some important background information from the journal Atrus has given you. Gehn is actually Atrus’ father, and Riven is one of the ages he has written. A D’ni supremacist (despite only being half-D’ni himself…a detail I’m sure he conveniently glosses over), Gehn is convinced that the Art consists of actually creating worlds, thus perceiving himself as the rightful ruler of any age he writes. He wants nothing more than to return to D’ni, so that he can have unlimited access to books and tools. Obviously, having a tyrannical megalomaniac wandering around unchecked isn’t a very desirable prospect; fortunately, due to a turn of events described in detail in the Myst books, Gehn is currently trapped on Riven. Problem is, Sirrus and Achenar have also sent Catherine there to get her out of their way, thus providing Gehn with a perfect blackmailing scheme. On top of that, Gehn is not a very good writer, and his ages therefore tend to be unstable. Riven is no exception and is actually rapidly falling apart, hence the name. Atrus’ writing is the only thing currently holding it together, which is why he couldn’t go rescue Catherine himself before and why he couldn’t come along for this trip either. Well, you’ve got your work cut out for you.