The narrator is a man completely captivated by the charm of mesmerism. We are presented with something truly out of the ordinary, a method that fully engages you, bringing you into a kind of trance. Those who practice it believe it's not just a mind game, but an actual gateway to a new self-understanding, promising significant beneficial effects. Then there's the encounter with M. Valdemar, a person suspended in a limbo between life and death, due to an incurable illness. This sparks an idea: why not take advantage of this tragic yet unique situation? Why not test mesmerism's effectiveness at the critical moment when life is about to leave the body? Valdemar, with an open mind and a touch of curiosity about science, agrees to participate in this unconventional experiment. He commits to signaling the narrator as his end approaches. When doctors announce his time is nearly up, Valdemar calls for him, ready to embark on his extraordinary and final adventure.

In the presence of doctors and an assistant, the narrator begins the mesmerism experiment on Valdemar, who starts to show clear reactions indicating the procedure's effectiveness. These signs include changes in his consciousness state, with Valdemar appearing to enter a kind of trance, variations in his breathing and heart rate, and a significant reduction in reactivity to external stimuli, thus indicating that the mesmeric techniques are indeed influencing his psychophysical state.

After several hours, Valdemar remains in trance, his body rigidly immobile but able to respond to questions through faint whispers. This trance state persists for months, during which the doctors observe him and find no signs of physical decomposition. After seven months, they decide to attempt to awaken Valdemar from the trance, but the experiment yields disturbing and disastrous results.

As the narrator tries to interrupt the mesmeric trance and return Valdemar to a normal state of consciousness, Valdemar himself, in a semi-conscious condition, makes an urgent plea. He begs to be quickly returned to a deep sleep state through mesmerism or, conversely, to be fully awakened. He says he feels dead, an experience he finds unbearable. But when the narrator tries to bring him back to the living world, something terrifying happens. Valdemar's body suddenly starts to decay, as if time were fast-forwarding. Before everyone's astonished eyes, his skin crumbles, turning into something repulsive. And there, under the incredulous gaze of the onlookers, Valdemar transforms, shifting from a kind of lethargy to such advanced decomposition that he becomes almost liquid. An ending so horrible that it leaves everyone speechless, a shocking testament to how unpredictable and terrifying nature can be.