Parasitic Capriccio begins in a near-future Tokyo where a new form of life has emerged: microscopic parasitic larvae that target the human nervous system, specifically the region governing artistic impulse and creative drive. The setting is a sprawling metropolis divided between sterile, controlled zones and decaying outer wards where the infection has spread unchecked. The central conflict arises not from the parasites causing death, but from a bizarre and terrifying transformation. Infected hosts, known as Maestros, develop an insatiable compulsion to create art, but their bodies physically warp to become the medium. Skin flakes become paint chips, hair strands stiffen into violin strings, and vocal cords mutate to produce frequencies that shatter glass and warp perception.

The story follows Ren Soma, a former child prodigy pianist who lost his hands in a lab accident and now works as a parasite containment officer. His primary weapon is a sonic disruptor tuned to the specific frequencies that calm or agitate the parasites. He is partnered with Kiri Utsuro, a senior officer whose body is a living cage for a dormant queen parasite, granting her the ability to sense Maestros from miles away but slowly erasing her own memories and personality. Their first major case involves a missing persons call in the Shinjuku Art Block, leading them to a former symphony hall now acting as a nest. There, they encounter the first major antagonist, a Maestro known as The Conductor, whose body has fused with the hall's pipe organ, allowing him to broadcast his parasitic influence across entire districts, turning ordinary citizens into chorus members who mindlessly hum a maddening tune.

Notable narrative arcs include the Fugue State arc, where Ren must venture into the no-go zone of Chiyoda to find a rumored cure before Kiri’s queen parasite fully consumes her identity. This arc reveals that the parasites were not an accident but a failed bio-weapon designed to weaponize culture. Another key arc is the Capriccio of the Masses, where a splinter group of Maestros, believing the transformation to be evolution, kidnaps a renowned composer to force him to write a symphony that will trigger a mass pupation event across the city. The narrative escalates into a psychological and body-horror thriller, forcing Ren to confront the possibility that his own desire to create music might be the very impulse that makes him vulnerable to the parasite. The story culminates in the Tokyo Opera House, where Ren must decide whether to destroy the parasite network and silence all art, or allow it to spread, birthing a new, monstrous form of human expression.