An earthquake causes a deep crevice to open up in the earth outside of a southwestern dirt farm. Out of the crack pours an army of deep-earth roaches with the ability to start fires by rubbing their cerci together. The sudden pressure change has caused their airtight bodies to develop a nasty case of the bends, so on top of their firestarting gig, they have a tendency to detonate. They torch a few houses and blow up some cars. We also get to see them zap the shit out of some humans.
The husband of one of the victims is a Professor of Entymology at the local university. Her death sends him over the edge and he isolates himself in a cabin for what feels like weeks (of story time, not of film time...well...) trying to find some way of killing them off. At least, I think that's what his gambit is supposed to be about. What happens instead is another shining example of Science Making Things Better.
Mating a normal cockroach with one of the firebugs, he creates a new and improved super-roach. The super-roaches are three times as big as their firebug predecessor, more responsive to pressure changes, and more aggressive. Oh yeah -- they can spell shit now too. Somehow the bugs have some communal mind and can line their bodies up in letter form. They spell his name, then "X" and "Y, Z" as he calls them out. Then, "WE LIVE."
It gets worse when they grow intelligent enough to start cross breeding themselves. By film's end, they sprout wings and fly. Doctor Moron manages to cause a cave in, trapping the little critters, through the ingenious scientific method of getting lit on fire and then falling into their cave, detonating on the way. It's really the only thing keeping him from taking home the Biggest Dipshit in the History of Mankind Award, and it's entirely unintentional on his part.
Say, cave in. Good idea. Why didn't anyone think to throw a stick of dynamite in there the moment they realized these things were packing heat rays?
It should be noted that Professor Bigshot is able to commit his colossal biological blunder largely because he has nobody supervising him. He sequesters himself in the house next to the roach hole and lives like a slob. Some peer assistance might have kept him from occasionally flying off the handle and deliberately blowing up a few cockroaches to avenge his crispy wife.
At least, he could have used the occasional reminder to lock up the cage containing the dangerous bugs. Dumbass forgets twice and the consequences are disastrous.
So let this be a lesson to all you kids out there trying to carve your initials in the tree of Mad Science -- come out for air and a shave every once in a while.