A small desert community is shaken by an earthquake and a large crevice opens up near a farm, from which come these ancient species of cockroach. These roaches are able to make fire. They make cars explode by crawling up the exhaust vents and set people on fire. Then they eat the carbon.
A local college professor (Bradford Dillman) goes to investigate after a former student tells him about the creatures and discovers that they are slowly dying of "the bends", as they came from deep beneath the Earth's surface and can't adapt to sea level atmosphere.
After his wife is killed by the bugs he goes crazy and is determined to keep them alive. He finds one of the last surviving roaches, builds a miniature pressure chamber, and puts it together with a common, household cockroach. They mate and the offspring are much more mobile - and carnivorous. The will only eat raw meat. The professor is more surprised when he wakes up one morning and sees that they escaped from their cage - and start spelling words on the wall...
This is a faithful adaptation of the book by Thomas Page (The Hephaestus Plague), who also co-wrote the script.