The narrator, who is only given the initial "P.," does not express surprise at the fact that everyone is so interested in the case of M. Valdemar, given the circumstances. Those involved have attempted to keep the affair quiet, but a confused and inaccurate account has nevertheless become public, leading to discussion and disbelief. As a result, the narrator wishes to clear up the facts as well as he can in his own account.
P. has for the last three years been interested in mesmerism, and nine months ago, he realized that no one has been mesmerized while on the cusp of death. The attempt would determine if such a subject is still susceptible to the magnetic influences of mesmerism, if the magnetic influences are decreased or increased, and if death can be temporarily prevented. P. decides to send a message to his friend M. Ernest Valdemar, a thin person whose white whiskers contrast with his black hair, since the narrator feels that the nervous man would be a good subject for such an experiment. The narrator has previously hypnotized his friend but has never achieved full control, probably, the narrator assumed, due to Valdemar's tuberculosis and declining health. He approaches Valdemar, who excitedly agrees, to the narrator's surprise. Since the doctors are able to predict the time of Valdemar's death, they arrange to meet about twenty-four hours before his predicted death.
When Valdemar sends for P., the latter goes immediately to Valdemar's bedside, where he observes that his friend has become emaciated and his pulse weak, although Valdemar retains his mental faculties and a small amount of physical strength. P. asks Doctors D. and F. to assess the extent of Valdemar's tuberculosis. They describe the damage and predict that he will die at about midnight on Sunday; it is currently seven o'clock on Saturday evening. D. and F. have already said goodbye to Valdemar but agree to come in at ten tomorrow to observe him.
When the doctors leave, the narrator explains the details of the proposed experiment to Valdemar, who is eager to make the attempt. P. decides that having only two nurses as witnesses might not be wise, should something amiss occur, so he decides to wait until eight o'clock on Sunday, when he brings a medical student, Mr. Theodore L., with him to the chamber. L. agrees to take notes, and at about 7:55, P. has Valdemar give his witnessed consent. P. hypnotizes Valdemar but accomplishes little by the time the doctors arrive. They agree that the patient is near death, so P. tries again, altering his technique for fifteen minutes until the patient's heavy breathing quiets. By 10:55, Valdemar is completely mesmerized, and Dr. D. decides to stay all night with the narrator, Mr. L., and the nurses. Dr. F. promises to return at dawn.
They leave Valdemar alone until three in the morning, and P. sees that Valdemar's body is still rigid and cold but not yet at the point of death. Valdemar's right arm readily follows P.'s commands, which is startling, considering that Valdemar has historically not been very responsive to mesmerism. He asks Valdemar if he is asleep, and after two unsuccessful attempts, Valdemar responds positively and asks P. to let him die without waking. P. asks if he feels pain in his chest, and Valdemar answers negatively and adds that he is dying. P. decides to wait until daybreak, when Dr. F. returns and is surprised that Valdemar is still alive. P. again speaks to Valdemar, who responds on the fourth attempt that he is still asleep but dying.
The doctors agrees that Valdemar should be allowed to die in his current state, since death will probably come in a matter of minutes, but the narrator asks the same question one more time. In response, Valdemar's eyes and mouth open to form a hideous sight, and his body no longer appears alive, but his tongue continues to move. In a harsh, broken voice, Valdemar says that he was sleeping but is now dead. At this, Mr. L. faints, the nurses flee, and everyone that remains silently tries to revive Mr. L., who awakens after an hour. They check on Valdemar, who is not breathing and has no blood flowing from his arm. The narrator can no longer move his body with mesmerism, but the tongue continues to attempt unsuccessfully to answer his queries, and no one but P. can be placed in "mesmeric rapport" with the patient. They find new nurses, and the doctors, Mr. L., and P. leave at ten in the morning.
In the afternoon, everyone returns to see Valdemar in the same condition as before. They decide that awakening Valdemar would lead to his complete death, so they keep him in this condition for seven months and visit him daily. On the Friday prior to the narrator's writing of his account, they attempt to awaken him. P. cannot influence Valdemar's arm but asks him about his feelings and desires; Valdemar answers, "For God's sake!--quick!--quick!--put me to sleep--or, quick!--waken me!--quick!--I say to you that I am dead!" Unsettled, the narrator attempts to awaken him, but the body shouts "dead! dead!" as it disintegrates into a liquid, rotting mass.
Although the purported intention of "The Facts in the Case of M. Valdemar" is to show how death can be arrested by the act of mesmerism, the story ends by suggesting that death is inevitable, in spite of all the efforts of human pseudoscience. Although the doctors' description of the deterioration of Valdemar's body is horrifying in its inclusion of details about pus-filled nodes and the turning of lung tissue into cartilage and bone, the gruesome nature of Valdemar's final transformation into a putrefying liquid is ultimately more frightening than Valdemar's natural death by tuberculosis would have been. As he approaches death, Valdemar becomes eager to try his friend's proposed experiment because he is apparently driven by the fear of death, but in his case, the postponement of death merely increases its finality.
By having the narrator explain that he wishes to dispel wild public rumors by providing as accurate an account as he can, Poe provides a sense of realism to his story. In fact, he published the story without an author, and a number of people confused the story as being a truthful, scientific account. Poe apparently took his inspiration from Dr. A. Sidney Doane's record of performing surgery on a patient who was in a state similar to M. Valdemar's semiconscious hypnotic state, and his seemingly clinical descriptions of Valdemar's illness lend to the air of authenticity. To a modern-day audience, "The Facts in the Case of M. Valdemar" reads purely as a lurid dramatic tale, but while Poe may have intended his story to be that way, he also recognized the story's potential as a practical joke. The scientific theories of the early nineteenth century did not contradict the possibility of the suspension of death through mesmerism, and Poe exploited these open-minded beliefs to craft his story.
In addition to the use of pseudoscientific realism and jargon, part of what gives the story its emotional impact is Poe's sensationalist approach to fear. In other pieces by Poe, the terror comes from the tension between the reader's lack of knowledge about details such as the narrator's identity or the history of the Gothic setting. In this story, by contrast, the setting is known, and Valdemar's background as a writer is innocuous. However, once the act of mesmerism commences, the minutely described evolving state of Valdemar's body creates a sense of increasing uneasiness which culminates in the semi-dead man's pleas to die and in the dissolving of his body. Not only has the narrator failed to prevent death, but he has also caused the body to suffer an even more repugnant fate.
In this story, Poe seems to indicate the existence of a divide between the body and the soul, as the body falls toward death but the soul remains somewhat intact. Long after the narrator can no longer move any of M. Valdemar's external limbs with the magnetic influence of mesmerism, the dead man's tongue continues to struggle to voice his words. Because it is both the mouthpiece of M. Valdemar's soul and a physical part of his body, it wavers between the absolute stillness of the body and the active desire of the soul for death. Even as the body is crumbling into liquid, the tongue is the last thing to disappear. The tongue continues to shriek, "dead! dead!" and thus suggests that although Valdemar is dead and his body gone, his soul still remains in some form.
Edgar Allan Poe is viewed as one of the forerunners of science fiction, and "The Facts in the Case of M. Valdemar" includes a number of elements that would later be developed in the science fiction genre. The narrator views his experience with M. Valdemar as a scientific experiment, and he tries to be an objective observer of the entire process, although Valdemar's unnerving end does shake him. Poe includes such details as scientific pseudo-jargon and an exploration of the implications of dangerous science in order to give the story an air of speculation for the future of science.