The bulk of the story concerns people living in a world which is dominated by giant automatic factories (hence the title) which are no longer under human control. A plot to destroy the factories nearly works, but they have one last trick up their sleeve ...
Every few moments, a pellet burst from the valve and shot up into the sky. The nozzle revolved and altered its angle of defection; each pellet was launched in a slightly varied trajectory.
"How far are they going?" Morrison wondered.
"Probably varies. It's distributing them at random." O'Neill advanced cautiously, but the mechanism took no notice of him. Plastered against the towering wall of rock was a crumpled pellet; by accident, the nozzle had released it directly at the mountainside. O'Neill climbed up, got it and jumped down.
The pellet was a smashed container of machinery, tiny metallic elements too minute to be analyzed without a microscope.
"Not a weapon," O'Neill said.
The cylinder had split. At first he couldn't tell if it had been the impact or deliberate internal mechanisms at work. From the rent, an ooze of metal bits was sliding. Squatting down, O'Neill examined them.
The bits were in motion. Microscopic machinery, smaller than ants, smaller than pins, working energetically, purposefully - constructing something that looked like a tiny rectangle of steel.
"They're building," O'Neill said, awed. He got up and prowled on. Off to the side, at the far edge of the gully, he came across a downed pellet far advanced on its construction. Apparently it had been released some time ago.
This one had made great enough progress to be identified. Minute as it was, the structure was familiar. The machinery was building a miniature replica of the demolished factory.
"Well," O'Neill said thoughtfully, "we're back where we started from. For better or worse ... I don't know."
"I guess they must be all over Earth by now," Morrison said, "landing everywhere and going to work."
A thought struck O'Neill. "Maybe some of them are geared for escape velocity. That would be neat - autofac networks throughout the whole universe."
Behind him, the nozzle continued to spurt out its torrent of metal seeds.
It's interesting to see how many "recent" nanotechnological ideas already occur in this story: