In a bustling New York City, Dr. Morris Ackerman returns to his apartment, bringing home treats for a peculiar guest—his wife Martha is excited to introduce him to "Elmer," a bizarre one-foot-long parasite with mesmerizing blue eyes. However, when Martha heads to the bathroom to feed Elmer, she discovers that he has vanished, sparking a frantic search throughout their home. In their chaotic state, both Morris and Martha succumb to an unusual foaming at the mouth.
Meanwhile, the couple's neighbor, Brian, is stirring from his sleep, reluctantly covered under the blankets. His girlfriend Barbara enters the scene, ready to rouse him for a concert. Feeling unwell, Brian suggests Barbara go with his brother Mike instead. After they leave, Brian drifts back to sleep, only to awaken later with blood trickling from a strange wound on his neck. Startled, he begins to hallucinate, envisioning vivid blue waters flooding his room and an ominous glowing eye peering down at him.
In the bathroom, Brian examines a gory injury on his neck, and suddenly, Elmer obediently slithers up his back. Elmer proposes a deal: if Brian will take him outside, he’ll provide him with euphoric sensations described as "light." Succumbing to temptation, Brian attaches Elmer to the back of his neck. Elmer then punctures Brian with his needle-like tongue, injecting him with an enticing blue substance that promises euphoric experiences straight to his brain.
Once outside, Brian embarks on an exhilarating adventure across a junkyard, overwhelmed by dazzling lights and colors. His escapade draws the attention of a security guard, whom Elmer inevitably attaches to, leading to a horrific scene where Elmer devours the guard's brain, leaving Brian reeling from the gruesome act. Soon after, Elmer insists on returning to the Ackermans' apartment.
Days pass, and Barbara grows concerned about Brian's erratic behavior, questioning Mike about it. As Barbara knocks on the bathroom door, Brian is taking a bath with Elmer. Later, at a dinner, Brian tries to confide in Barbara about Elmer, only for the parasite to bite him while concealed under his clothing. In desperation for more of Elmer's substance, he finds himself stumbling into an underground rock club named Hell, where a beautiful blonde woman draws him outside and seals their fate with a kiss. Elmer takes this opportunity to attach to her, draining her life away.
Regaining clarity, Brian is horrified by the blood that covers him. His alarming encounter reaches a peak when Morris confronts him, revealing the dark history of Elmer, known as "Aylmer," a creature with a lineage stretching back to the Crusades. Panic-stricken, Brian flees from Morris and returns to his hotel room, placing Elmer in the sink while seeking answers about Morris. Elmer tells him chilling truths: that he was deprived of human brains in favor of animal ones, which kept him weak, while also clarifying his insatiable hunger for brains.
Amidst a spiral of addiction and existential dread, Brian finds himself detoxing from Elmer's blue serum, agonizingly begging for more. In a grim turn, Elmer again compels Brian to find a fresh brain. Sneaking into the hotel's bathroom, Elmer claims another victim in a shocking act of violence.
Back at their apartment, a tense situation arises when Mike and Barbara, searching for Brian, connect in an intimate moment. Disturbed by their closeness, Brian attaches Elmer to himself again and is engulfed by a wave of violent fantasies involving Barbara. After a confrontation, Elmer attacks Barbara, resulting in her tragic demise.
Outside, Morris confronts Brian, leading to a tragic standoff where chaos ensues. Elmer, ever the predator, attacks Morris as he tries to save Martha, ultimately resulting in her death as well. In a messy culmination of betrayal and horror, Brian and Elmer sustain their bond through mutual dependence, leading to catastrophic consequences that leave Brian dangling at the edge of his sanity.
As the climax unfolds, Mike enters to find a distraught Brian, who, caught in the throes of despair, grasps a gun to his head. The surrounding tenants alert the police to the unusual racket. The sound of a gunshot fills the air, leaving the onlookers to witness Brian's grotesque fate as his head reveals an eerie light pulsating from within, marking a haunting conclusion to this tale of addiction, horror, and the grotesque temptations that lurk in the shadows.
