Marwella Harbison Helen Hughes is an eccentric old woman who tends a greenhouse full of unusual plants. Among her specimens is a striking yellow bloom from a newly formed volcanic island off Micronesia, a plant that seems to carry an unsettling energy. A local handyman, Fre d Adams Sandy Webster stops by to fix the light fixtures and notices the drooping plant. He jokes that the plant has no thorns, yet when he accidentally pricks his finger, an odd numbness spreads through him. After saying goodbye to Marwella, he heads to his truck, only to collapse almost immediately, overwhelmed by a sudden fever.
Marwella calls for help, and a paramedic arrives to transfer Fre d to Hill Valley Hospital. Inside the hospital, two doctors—Dr. Rachel Carson Gwynyth Walsh and Dr. Judith Glass Susan Anspach—are astonished to discover that the prick has already begun to produce dangerous gangrene. Their attention shifts when a gunshot wound victim, Det. Jim Bishop Steve Railsback, is brought in with a partner who took a bad hit during a stakeout. Rachel remains focused on the surgical path to save Jim, even as the emergency room buzzes with alarming developments.
In a nearby bed, Fre d convulses and vomits a bizarre insect pupa. The creature is quickly sealed in a bell jar in the hospital’s in-house lab, while Rachel worries about the possibility of contagion spreading through the staff and patients. Jim, who is asked to monitor his own exposure, emerges from testing with a clean bill of health, allowing Rachel to guide him on a tour of the facility, including their high-tech laser research labs. Yet x-rays offer no clear answers, so the team opts to surgically slice the pupa open. The moment they do, a fierce insect emerges, forcing Jim to help corral the creature back into containment.
The situation grows grimmer as Fre d’s mysterious illness worsens. The parasite infection rides along with the growing danger, and Fre d experiences a sudden cardiac arrest; Judith Glass attempts to revive him with shock, but his chest erupts violently. Marwella and the paramedic who brought Fre d to the hospital fall ill as well, signaling that the threat is spreading.
Hospital director John Vernon Roger Levering resists quarantining the building in fear of triggering a panic, but Rachel convinces him to summon entomologist Elliot Jacobs Don Lake to help identify the insect menace. Before help arrives, a group of children from the pediatric ward wander the hallways and sneak into the lab where the insect is kept. Lab technician Alice Bradley Cynthia Belliveau is distracted by her boyfriend, laser lab technician Ted Andrews Dan Lett, and the insect is left unguarded. The curious children encounter the creature and, mistaking a bluish powder on a shelf for a harmless substance, pour in an excess amount.
The insect grows to a terrifying size, escapes the glass enclosure, and promptly kills the nurse and an orderly who were guarding the lab. Elliot Jacobs arrives and, alongside Rachel and Jim, witnesses the carnage. He is horrified to learn that the bluish powder is a growth hormone that enabled the insect to swell to monstrous proportions. The trio realizes the danger is now underground in the hospital’s utility tunnels, where the creature begins to lay its brood.
As the scientists scramble to devise a stopgap, Judith Glass confronts the human fallout of the outbreak while the military—summoned by the Lincoln Institute, a facility dedicated to disease prevention—takes drastic measures to prevent a broader catastrophe and, in the process, destroys the infected hospital complex to contain the threat. The layers of containment, science, and frontline courage collide in a fight to keep the outbreak from spreading beyond these four walls.
