Planescape: Torment 's story begins when the Nameless One wakes up in a mortuary. He is immediately approached by a floating skull, Morte, who offers advice on how to escape. Morte also reads the tattoos written on The Nameless One's back, which were inked there as reminders to himself, that contain instructions to find a man named Pharod. After a conversation with the ghost of his former lover, Deionarra, and passing by various undead, The Nameless One leaves the mortuary to explore the slums of Sigil. He finds Pharod, who is the chief of an underground village of scavengers, and retrieves a magical bronze sphere for him.In return, Pharod gives him further hints to piece together his forgotten past. Later on, The Nameless One learns from a powerful sorcerer named Lothar that the night hag Ravel Puzzlewell caused his immortality, but the hag is currently imprisoned in a magical maze by the Lady of Pain.The Nameless One finds a portal to Ravel's maze, but realizes that it requires a piece of Ravel to activate it; for this, he locates Ravel's daughter and takes drops of her blood.

Once in the maze, The Nameless One converses with Ravel, who asks him, "What can change the nature of a man?" — a question that plays a prominent role throughout the game. Ravel is pleased with The Nameless One's answer because he offers his own thoughts; she claims she has killed many men in the past who, instead of giving their own answers, tried to guess what her answer might be. As the conversation progresses, Ravel explains that, in a past life, The Nameless One had asked her to make him immortal; however, the ritual she performed was flawed, causing him to lose his memory each time he died. She reveals that the mortality she separated from him was not destroyed, and that as long as he was alive, his mortality must still be intact. She does not know where his mortality is, but suggests that the fallen deva Trias might.

Ravel then attempts to keep The Nameless One there by force. After the Nameless One and his party leave the maze, Ravel gets up, having actually survived the encounter. The Transcendent One appears, and, after a short conversation, kills Ravel. Following this, The Nameless One travels to the city of Curst, a gate town on the border of the Outlands and Carceri, to meet and free Trias. Through a tip from Trias, who claims not to know where The Nameless One's mortality lies, The Nameless One then visits the Outlands and Baator, where he learns that his mortality lies in the Fortress of Regrets and that only Trias knows how to access this place. Meanwhile, however, the city of Curst has "slid" from the border of the Outlands to the neighboring chaotic plane Carceri due to the chaos unleashed by Trias after The Nameless One freed him. After a fight, Trias tells The Nameless One that the portal to the Fortress of Regrets is located in Sigil's mortuary, in the very room where the game began.

In the Fortress of Regrets, The Nameless One encounters three of his past incarnations: one practical, one good, and one paranoid. The Nameless One learns that the "good" incarnation is the original, who was made immortal by Ravel.The Nameless One had committed immeasurably terrible deeds in his lifetime, and when he realized there would be retribution on his soul when he died, he sought to postpone death as long as possible in order to right his wrongs. After meeting his past incarnations, The Nameless One confronts his mortality—embodied as a powerful being called The Transcendent One. The Transcendent One reveals that since being separated from The Nameless One, he has enjoyed his freedom and has been attempting to erase clues that might lead The Nameless One to discover the truth. Depending on the player's choice, The Nameless One either slays his mortality or convinces it to rejoin with him; either option finally ends his immortality and allows him to die. In the game's final scene, The Nameless One awakens near a battleground of the eternal Blood War between demons and devils; he picks up a mace and walks toward the conflict.

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