Atrus calls the player to his home in Tomahna to request his friend's assistance. Atrus is the writer of special books, which serve as links to worlds known as Ages. Twenty years earlier, his two sons, Sirrus and Achenar, destroyed his library on Myst and trapped their parents in order to plunder the wealth of Atrus' Ages. The player's intervention saved Atrus, who had imprisoned his sons via traps intended for thieves. Atrus' wife Catherine hopes that, after twenty years, they have finally repented for their crimes. Atrus is not as sure his sons have reformed, and so wishes the player to act as an impartial judge. After being knocked unconscious by an explosion, the player realizes that
Yeesha, Atrus' daughter, has disappeared. The player sets out to find Yeesha, traveling to the prison Ages of Spire and Haven. On Spire, Sirrus has used his scientific knowledge to craft explosives, allowing him to breach the chamber that contained the linking book back to Tomahna, and has escaped; his journals reveal furthermore that he doesn't feel any remorse over his past crimes. Journeying to Haven reveals that Sirrus has also freed Achenar, who, unlike his brother, has begun to live in harmony with the surrounding nature causing him to regret his respective crimes. After Spire and Haven, the player journeys to the Age of Serenia and encounters Achenar, holding a "Life Stone"; Achenar tells the player that Sirrus is mad and has captured Yeesha, reveals that he kept a journal from twenty years earlier hidden on the island, and warns the player not to let Atrus come after them. Achenar's journal reveals that he and Sirrus planned to trap their mother Catherine on Riven and use a "Memory Chamber", a gigantic flower-like structure used to preserve the memories of the dead, to take control of Atrus' body and steal his knowledge of the Art of Writing. The Life Stone that Achenar stole is used to power the Memory Chambers, leaving the current one in danger of collapse. Shortly afterwards, the player finds Sirrus in an underwater harvester used for collecting memory globes for storing those memories; he blows up the harvester and flees to an older Memory Chamber, decrepit and abandoned. After encountering the player there, Sirrus tells the player that Achenar is the guilty one, and asks the player to find Atrus and bring him to Serenia to set things right. Finding that the old Memory Chamber door has been locked by Sirrus with a special color-code combination, the player goes to the active Memory Chamber to seek aid from the Serenian Protectors, who believe that the answers can be found in their "mirror realm", known simply as Dream. Obtaining a "spirit guide", the player enters Dream and interacts with their guide, who tells them to interact with the Ancestors, the spirits of all Serenians who have died and had their memories preserved, to bring them into harmony. After bringing the Ancestors into harmony, the player discovers the combination to Sirrus' color-code lock. Returning to the "waking world" and entering the old Memory Chamber, the player finds Yeesha strapped into a chair, and she begs to be released from it with a silver lever. At that moment, Achenar arrives with a crossbow and the Life Stone, and warns that Sirrus used the Memory Chamber to remove Yeesha's memories and transfer his own into her body; Achenar points to an amber lever, which will reverse the mind-transfer. At this point, as in the other games, the ending
varies. Delaying too long will result in Yeesha (who is in fact Sirrus) taking Achenar's crossbow and shooting first him, then the player. The silver lever will release Yeesha (again, possessed by Sirrus), who kills Achenar and the player. In the good ending, the player pulls the amber lever, reversing the mind-transfer process. But because of the age of the Memory Chamber, it becomes unstable; Achenar tells the player to return to Dream and set Yeesha's memories right, while he uses the Life Stone to stabilize the chamber by inserting it into the chamber's shrine, poisoning him. In Dream, the player finds a monstrous creature, representing Sirrus' Dream-form, anchored to Yeesha's essence and preventing her from returning to her body; with no spirit guide, Sirrus is forced to cling to Yeesha to avoid being lost forever. He maintains his anchors by jumbling up Yeesha's memories. The player restores Yeesha's memories and frees her from Sirrus' grasp; Sirrus' Dream-form is destroyed by the shifting waves of Dream, killing him. The player awakens to find Achenar, fatally poisoned, confirming that the transfer was successful; he dies shortly afterward. The player returns to Tomahna to meet with Atrus, who says that Catherine has taken Yeesha to Tay (the "rebel Age" used to evacuate Catherine's people in
Riven), and remarks that while his sons are gone, his daughter is safe. ==Development==