Trelkovsky, an unassuming and quiet man, finds himself renting an apartment in Paris, previously inhabited by the troubled Egyptologist, Simone Choule, who tragically attempted to take her own life by leaping from the window. As he prepares to move in, he meets the building's concierge, who ominously reveals the location where Simone fell, leaving Trelkovsky with an unsettling curiosity about her demise.
His first encounter with Simone occurs at the hospital, where she lies bandaged and unable to communicate. During this visit, he meets Stella, a friend of Simone's, who is overwhelmed with grief and attempts to converse with Simone, only to witness a hauntingly terrifying scream escape Simone's lips. Following the hospital visit, Trelkovsky and Stella share a moment, ultimately leading to a tender encounter before parting ways. However, tragedy strikes when he learns of Simone's passing soon after.
As days pass in the apartment, Trelkovsky faces unrelenting criticism from his neighbors and the landlord, Monsieur Zy, who incessantly accuse him of disturbances such as throwing parties and excessive noise. Increasingly anxious about his unsettling living situation, Trelkovsky finds the apartment profoundly disturbing; he notices peculiarities like neighbors standing eerily still and a bizarre human tooth hidden in the wall. Despite sharing his concerns with friends, they dismiss his feelings, leaving him isolated and skeptical of his surroundings.
A mysterious visitor named Georges Badar, who harbored feelings for Simone, seeks solace and understanding from Trelkovsky. Together, their bond deepens as they recount memories of Simone, though Trelkovsky is noticeably haunted by the ghost of her presence. Wandering through a café Simone frequented, he becomes painfully aware of the lingering memories of her life, enduring pressure to adopt her habits, from ordering her favorite meals to consuming cigarettes she once smoked.
Trelkovsky's mental state deteriorates as he faces harassment from his neighbors, culminating in a traumatic burglary that leaves him on edge. As paranoia begins to seep into his mind, he wakes one morning to find himself eerily adorned, as if someone were attempting to mold him into Simone. Dressed in her clothing, he remains motionless in the dark of night, grappling with a terrifying thought: are his neighbors conspiring to lead him to the same fate as Simone?
In a moment of desperation, he turns to Stella for a reprieve but soon suspects her involvement in the sinister plot that he believes his neighbors have orchestrated. In a fit of madness, he vandalizes her apartment, driven by an overwhelming sense of betrayal. The oppressive anxiety culminates when he is struck by a car but manages to escape with minor injuries. Seeing the elderly couple responsible as manifestations of his landlord and the conspiracy surrounding him adds to his palpable paranoia.
Ultimately, Trelkovsky's fragile mind succumbs to the haunting whispers of fate, as he dresses once again as a woman and emulates Simone's final act of desperation. Amid an imaginary crowd of supporting neighbors, he leaps from his apartment window, only to be revived momentarily and restrained by onlookers. In a tragic echo of despair, he takes the plunge a second time just as the police arrive, sealing his fate.
In a haunting conclusion, Trelkovsky awakens in a hospital bed, bandaged similarly to Simone, trapped in a cycle of torment. His perspective relives that fateful day when he visited Simone, culminating with a chilling scream, reflecting the abyss of despair that envelops him in a tragic cycle of identity and loss.
