In the darkness, a single electronic eye blinked. A mote of metal, riding on a puff of nuclear flame, had violated the ancient boundaries. Systems that had laid dormant for millennia woke as power flowed into them. Powerful sensors reached out through millions of miles of the void, bringing the tiny ship into crystal-clear focus. An impassive command is called, and in answer two dozen missiles hurtle forth. In a blinding flash of light, the mote is gone.
* Time Passes *
The eye awakens again. Now three motes of metal approach, their course directly for the planet that the eye orbits in its eternal vigil. The ancient structure rumbles as missiles are loaded and fired, a single volley for each mote. Beams of coherent light reach out from the ships, grasping for the missiles that hurtle towards them. One, its programming slightly degraded by the eons of disuse, falls to the crude weapons - the rest do not. Nuclear fire tears into the ships, vaporizing metal and flesh. However, once the explosions have cleared, the motes remain. They are battered things, bleeding oxygen into the void. They turn around, attempting to flee from their fate, limping along at whatever speed their remaining engines can produce. A second volley of missiles is sent, and the ships stop their flight.
* Time Passes *
Six motes now violate the ancient borders. The aether around them boils with protective energy as they speed towards the Guardian. Missiles fly out through the void, their sleek forms promising death. Once again, the ships fight back, their beams of light firing more accurately now. Explosions pock the incoming swarm, and half a dozen are stopped short of their goal. The rest pounce upon their prey, swatting away the protective energies and digging deep into the metal beneath. The first volley fails to stop the intruders, as does the second, and even the third. The fourth, however, finds only broken metal and bodies floating in space.
* Time Passes *
Seven ships approach; three of the same type that had previously failed, four displaying the increased size and signature returns that signal more advanced shields and additional weaponry. Missiles fly and space boils with coherent light, metal slugs, and atomic fire. Armor buckles and breaks, engines explode. Three ships explode into wrecks, while two more stop dead in space, all engine activity dead. Only two ships remain on course, limping through space towards the planet. The artificial mind prepares to annihilate these final bastions of resistance... then stops, as error messages glow red in its mind - the magazines report only a single load of missiles remain. Desperate calls are made to the manufactory, to collier ships, to the armament pits... all return timeout errors. The Guardian is on its own.
The ships close to within mere tens of thousands of kilometres. The beams of light - so ineffectual against the missiles - burn through the protective plating of the Guardian, scoring deep hits and damaging internal systems. Sensing that the time is right, the final salvo belches forth at minimal range. The targeted intruder craft buckles under the annihilating fire, its systems go dark, and its puny biological crew dies.
However, one ship remains - and its weapons retain their bite. Salvo after salvo punch through armor. Distress signals go out, unheeded. The Guardian's electronic eye blinks, then shuts forever; its vigil over the world it was entrusted has ended.