When crude oil waste from Global Chemicals seeped into a mineshaft in south Wales, several maggots were contaminated. The waste triggered atavistic mutations that caused the maggots to grow to enormous proportions. The giant maggots are about three feet in length, with plump, shiny grey bodies that are completely covered by thick chitinous plates. At one end of their bodies is a mouth, lined with teeth and crowned by a pair of fangs. They usually move by inching along the ground (although some can move more quickly) but they can also climb walls and jump quite a distance from the ground to attack their prey. When they move, they leave a trail of slime behind them. The maggots hiss quite frequently, sometimes when approached by a living creature and sometimes just because they feel like it. They can also be distracted and disoriented by the Doctor's sonic screwdriver.
They begin life in a large, silvery egg which then hatches into the maggot. Eventually, given a sunny environment, they pupate, and out of the chrysalis hatches a dragonfly-like insect. This creature has bright red eyes, vicious mandibles, and a long thin body with clear wings. It can shoot a spray of green venom - probably the same slimy substance excreted by the maggots.
The maggots used the mine as a breeding area, saturated as it was with the oil waste. This waste was a bright green slime that burned to the touch and smelled like something rotten. The slime was also hazardous - anyone who touched it would develop a glowing green patch of skin that would spread until the victim died. Similarly, being touched by one of the maggots would cause the same effect. This was because the maggot cells changed the internal structure of human cells into their own nature, much like a virus, and they attacked every cell in the body. Antibiotics can slow the spread of the infection, but cannot stop it completely. The cure is a variety of fungus which not only rapidly stops the infection, but will also kill any maggots that eat it. It is unclear what exactly the maggots are supposed to eat - they attack humans but never consume them, but they clearly do need nutrients as shown by their ill-advised attempts to eat the fungus.
The army tried to seal up the mine by blowing it up, but instead this caused the maggots to crawl to the surface through the old shafts, in order to bask in the sunlight on a slag heap and pupate. Other methods were tried to kill them, but pesticides had no effect, and their chitinous plating protected them from explosive charges. Eventually they were killed with tempting fungus handouts, but not before one of them had pupated. Lest the insect spread the maggot infection around Britain, the Doctor was forced to kill it with his coat. The army subsequently cleaned up the slag heap, tossing the dead maggots into burlap sacks for disposal.
