Description
Marwellia Harbison is an eccentric old woman who owns a greenhouse. Handyman Fred Adams stops by to do some work on the light fixtures and comments on how one of her plants is drooping. It's an odd looking plant with yellow flowers. According to Marwella, it came from a newly formed volcanic island off the coast of Micronesia. Its current state happened overnight and Marwella is at a loss to explain what could have caused it.
Fred inexplicably hurts himself on the plant, as though he pricked his finger on a thorn. However, Marwella claims the plant has no thorns and neither she nor Fred can figured out of what could've pricked him. Nevertheless, he seems okay, and after bidding Marwella goodbye he walks out to his truck whereupon he immediately starts feeling strange and collapses. Marwella calls the paramedics. Fred is taken by ambulance to the Hill Valley Hospital with a very high fever. On call are Doctors Rachel Carson and Judith Glass, who are astonished to see that the man has already developed gangrene where he pricked his finger. Their attention is taken away from Fred when police detective Jim Bishop brings his partner in with a bad gunshot wound. The two had been involved in a stakeout that went poorly, and Jim's partner got shot at point blank range. However, Rachel is confident that with surgery, he should to be okay.
In the adjacent bed, Fred begins convulsing and winds up vomiting an insect pupa out of his mouth, after which he seems to stabilize. The pupa is hurriedly contained in a bell jar in the hospital's in-house laboratory. Rachel is concerned that whatever Fred had might be contagious and orders mandatory checkups of everyone, including Jim. When Jim comes back clean of any mysterious parasitic insects Rachel decides to show him around the hospital, including their high-tech research facility where they're testing out new and powerful surgical lasers. Attempts to x-ray the pupa prove futile, so it is decided to slice it open. In doing so the doctors unleash a particularly feisty insect-like creature, but with Jim's help they're able to get it contained. Meanwhile, Fred is faring worse. In addition to having been parasites from the insect, he's come down with a mysterious disease the creature was carrying. He goes into cardiac arrest and when Judith Glass attempts to revive him with shock paddles, his chest violently explodes. Meanwhile, Marwella and the paramedic who brought Fred in are coming down with the same symptoms.
Hospital director Roger Levering is resistant to quarantining the facility for fear of causing a panic, but Rachel is at least able to get him to bring in entomologist Elliot Jacobs in the hopes of identifying the mysterious insect. Before Elliot arrives, a group of children from the hospital's pediatric ward start roaming the halls. Lab technician Alice Bradley, tasked with guarding the insect specimen is lured away from her post by her boyfriend, laser lab technician Ted Andrews, to have sex, which leaves the insect completely unguarded. The children wander on into the lab unnoticed. Seeing the insect, they decide to feed it some bluish powdery substance one of them finds in a bottle on a shelf. They argue over who gets to pour it into the jar with the insect and, as a result, they pour in an excessively large amount of the powder. Hearing the nurse and orderly returning, the children quickly leave. Unbeknownst to the two, the insect has grown to gigantic proportions and broken free of its glass prison. It promptly kills them both.
Elliot Jacobs arrives and he, Rachel and Jim survey the carnage. Elliot is horrified when he discovers that the bluish powder the children fed the insect was a growth hormone. Now the insect is giant and it could be anywhere. It turns out the insect is in the utility tunnels underneath the hospital, where the creature kills a hapless janitor and then begins building a nest for its brood. Rachel, Jim and Elliot figure out how to stop the insect while Judith deals with the consequences of the disease the creature has spread throughout the building, before the military, summoned by New York's Lincoln Institute (a facility for disease prevention), take drastic measures and destroy the facility.
