Several months after the conclusion of Myst, Atrus asks The Stranger for his/her help (which was earlier mentioned in the ending of Myst, but never fully explained) and sends him/her to Riven to free Catherine and capture Gehn. He hands him/her a Trap Book, disguised as a link to D'ni, and his journal and explains that he can't send you in with a way out, otherwise Gehn could potentially escape if (s)he fails. He asks him/her to signal him somehow and sends him/her to Riven.