I can smell my nemesis, see the residue of his presence on the fallen buildings. I am getting closer. This city is in ruins like all the others, but I can sense life somewhere in the shadows; the flickering heartbeat of fusion cells. Organic life is present too, faint hints of humanity too recent to be from before the war. Duty calls me onward, sniffing rubble and stalking through the sewers. I pause to watch for movement, but all is silent and still. A rodent, unperturbed by my cat-like appearance, climbs onto my leg and begins to groom himself. I have no orders regarding rats.
He has hidden himself well - impressive considering the entourage of humans. The organics are sloppy, easy to track. For the first few years after the war they seemed to throw themselves in front of our weapons. I wonder if this recent difficulty heralds some thinning of the herd, where my fellow cleaners have destroyed the slow and careless and left the society to form out of quiet, discreet beings. Doubtful, since there has barely been enough time for a single new generation to come of age - still, the fact that I am already having problems in following them is both impressive and disturbing. It could be that there are less than the twelve I had estimated.
Proceeding down into a valley caused by the collapse of the underground transit systems, I find a shoe with fresh DNA on it. So close. My nemesis is nearly within my grasp. I have watched traps and ambushes kill off the others from my squadron, crush them before they could see our duty fulfilled and the last traces of the United Americas destroyed. I have received no signal from Eurasia, no communication from other Cleaner crews further south. This may be my burden alone, and I cannot fail. I can smell the humans on the breeze, flowing from a half-collapsed tunnel. Their time is up.
I circle around to get ahead of them and I wait. A heavily damaged IR81 is slumped against a wall nearby and I want to finish it, rip the reactor from its frame, but I force myself to remain hidden. Its ruined legs aren't taking it anywhere, I can return once I have ambushed the humans. An hour passes, and I hear them coming closer, smell them on the air. I can even sense the energy signature of my nemesis. More time passes and I realize something is wrong; they are moving too slowly, and making far too much noise. I charge towards it and break through into the plaza... and find a decoy.
The Highway Assembler is draped in human's clothes, and rumbles along the street acting on some sort of pre-programmed instructions. Once again my nemesis has made a fool of me. I run as fast as I can back to the valley and beyond, searching for them. They have bought more time, but it will not be enough. I will find them, will destroy them. On all fours, running at full speed, I see the tripline too late - cartwheeling for twenty feet before I slam into a wall. A metal rod, torn from the foundation of some building, slams into my right shoulder and expertly slides between armored plates, tearing vital wires and crippling me. I welcome this attack, because it means I am finally face to face with my tormentor after all this time.
He looks terrible. He is military, but outdated; a CM125 without any upgrades. He hurls a block of stone at me that I easily dodge as I pull the metal rod free - I long to launch a missile into him but ammunition has been so hard to find. This fight will be up close and personal. My first thrust falls short as I realize my leg is caught on debris, and he grabs the weapon so that we are both holding it. Pivoting, he jams the barrel of his gauss rifle into the side of my head - damaging both - and then falls as I sweep out with my free leg. I fall as well, but it is planned and I am able to dart my head forward into his neck - my jaws clamp down and sever a pneumatic line that would have been better protected in a newer model.
He thrusts something up against my thorax and I feel a concussive blast - some sort of jury-rigged bomb has peeled back my plating and removed my nemesis' hand. For the moment, neither of us makes a move. I am slightly more damaged, but am also faster and stronger. I need only to restrain him for a moment in order to deliver a killing blow.
"You don't need to do this," he says, "there's no war anymore. Nobody left to be at war with."
"There is you." And after him there will be his humans, and then the disabled robot and highway assembler, and then I will move on to the next city and the next until none remain.
"The Americas had Cleaners too, you know. We were too good - Oceania is gone, and if there's anyone left in Eurasia I'll be surprised. This is it. The humans I'm protecting might be the last ones."
I tell him that I'm aware of this. He is correct, after all. I find it to be unlikely that the ones who built me survived the onslaught.
"Then why? Your programming must allow for some change of situation, some option for you to abort if the conditions are altered."
"It does," I tell him, "But I choose to do my duty." I lunge at him and spear the metal rod through his chest - it angles upwards just as I intended and cracks his reactor casing; he will be forced into emergency shutdown.
"I choose the same," he says, and the world goes white.
He had another bomb, a larger one. I have no sensory input save for optical in the visible spectrum. I can see parts of me lying thirty feet away, but there is no trace of my nemesis. I can take some comfort in the fact that without him the humans stand no chance; there must be only a handful of them. I had already lowered my estimates, and now after seeing the shape he was in I suspect their numbers are even less. Possibly five. My reactor is stuttering, and I know that soon I will shut down. From the shadows of the city steps a single child. Was this it? Did he really destroy himself to protect one malnourished girl? A hand rests on her shoulder, and I realize there is someone else. Everywhere, between the vines and the twisted stone and metal, I can see pale limbs. How is this possible? They march forward, appearing from the wreckage without a sound to reach for me - a hundred silent and vengeful ghosts.