On the fortieth day of his journey, XKD-47 came across a unit like himself, buried almost waist-deep in sand. Her datacast was badly corrupted by damage and corrosion – clearly, she had been caught in the storm and had spent every moment since staring hopelessly at her destination, utterly immobile. She’d been crossing a teetering walkway over a smooth-sided ravine when the storm had struck, and had been left utterly exposed.

While wilful deviation from his journey was not expressly forbidden, XKD-47 had felt more than a little conflicted the first time he’d spotted another unit on the horizon. It had taken him nearly a day to reach them through the labyrinth of fallen stone only to find that they’d been damaged far beyond his ability to repair. Ultimately, he’d been left with only one feasible course of action.

This was the fourth unit he’d found in a similar condition, and It wasn’t getting any easier. Reaching gingerly inside the mottled carapace of the helpless soldier, he felt around gingerly, trying to lessen the pain, until his questing fingers located the main power junction.

In the instant before the unit completed her journey, her battered face curved slightly in an expression of profound gratitude, and then XKD-47 was alone in the ruins once more. Turning cautiously, balancing himself on a creaking metal beam with both arms outstretched, he reoriented himself with his destination and moved—

The metal beam twisted treacherously underfoot, and sent XKD-47 tumbling into the abyss.

*             *             *

On the forty-first day of his journey, XKD-47 was in a hole.

*             *             *

On the two-hundred-and-twelfth day of his journey, XKD-47 was in a hole.

*             *             *

On the three-hundred-and-fourth day of his journey, XKD-47 drove the final piton into the silicon wall, placed his foot gingerly upon it and hauled himself bodily out of the hole. He sat for a while scanning the datacast channels, hoping that other units had completed a map of the area while he’d been trapped, but all the links were suspiciously quiet.

Gradually, XKD-47 began to believe that he might be the only unit left. It was a concerning thought, but the Commanders had been wise enough to give even a single unit enough destructive ordinance to win the war.

Standing unsteadily on rust-stained legs, he reoriented himself towards his destination, the stronghold of the Enemy, and began to move.

*             *             *

On the three-hundred-and-sixth day of his journey, XKD-47 was startled by a sudden flurry of sand and grit ahead of him. For a moment he thought the storms were upon him once more, but as the winds died down with unnatural speed, ill-defined shadows in the storm became familiar shapes.

The door of the blue box opened and the Doctor strode out into the ruins, looking as timeless as ever. “Now stop messing about,” he said by way of a greeting, adopting the demeanour of an exasperated schoolmarm. “Why haven’t you gone home, eh?”

“Why would I?” XKD-47 replied. He was surprised by the sound of his own voice; raspy and distant as though he were shouting through a length of pipe.  “My mission hasn’t changed.”

“Yes. No. But it has, though.” The Doctor looked uncertain for the first time since XKD-47 began his journey. “I fixed it all! Organised a cease-fire. Turned out it was just a big misunderstanding – their communication system jammed your engines and you thought you’d been shot down. Two technologies turn out to be incompatible, it happens sometimes… and you don’t believe a word I’m saying, do you?”

With that, the Doctor tugged a slender, silvery device from his pocket and began to sweep it back and forth over XKD-47’s pockmarked skull. It went swizzywizzywizzy.  “So the question is, why didn’t you receive the change of orders? Let me see…”

XKD-47 swatted the device out of the Doctor’s hand, irritably, and it landed in the rubble a short distance away making an indignant beeping sound. “I was down a hole,” he admitted, “for a long time. I had to wait for suitable raw material to fall in before I could escape.”

“So you never heard the cease-fire… Stan, the war is over. Your people are packing up to go home – I’ve helped them fix their engines, they’ll be gone by the end of the day. All you’ll be doing is—“

“Completing the task for which I was created,” XKD-47 snapped.  “Out in the wasteland are dozens of units just like me, trapped and lost in service to their orders! We fall and we stand, over and over, because we share a common purpose – a reason for our very existence!”

The fingers on his replacement hand flexed in and out, impulsively, and now words tumbled out of him in a fountain of rage and frustration. “Purpose burns within us and drives us onwards with our last ounce of strength! It is a noble existence, it is a pure and simple understanding of our place in things and for me to arrogantly claim a higher authority would be wrong!”

They stood in silence for a moment, and the Doctor said softly “Such a waste. You should have been a poet, not a bomb.”

XKD-47 lunged forward, suddenly, grabbing the Doctor by his jacket. “And yet a bomb is what I am!” he hissed. “You will stop attempting to deceive me, and you will instead use your travelling box to take me to the heart of the Enemy stronghold so that I can complete my mission.”

The Doctor shook his head, calmly. “No,” he said, and that was that.

“If you don’t…” XKD-47 hesitated. “If you don’t, I’ll detonate here and now. You’ll be destroyed.”

“But nobody else will be,” the Doctor replied, still with an infuriating calm. “And that’s my purpose. To help people. Even you, if you’ll let me.”

They stood, deadlocked, for almost a full minute, and then XKD-47 released his hold, turning once more to his destination.  “Go away, Doctor,” he said, setting off on his journey once again. “Get back in your box and choose to go away.”

The Doctor followed him a short distance, retrieving his sonic screwdriver. “I’ll have to tell them, you know,” he said, and suddenly he sounded very old and very tired.

“I know. Perhaps they may be able to do something for my brothers and sisters.” XKD-47 paused.  It was becoming harder and harder to speak with any clarity; his failing systems simply couldn’t take the strain. “I… regret… that you could not fulfil your purpose.  Thank you for my name.”

This time, it was the Doctor who walked away from the conversation.

*            *             *

On the three-hundred-and-seventh day of his journey, something flew over XKD-47’s head and landed noisily in the dirt ahead of him. It looked like a big grey egg, and a few moments later it split neatly around its circumference to reveal another unit. It clambered out nimbly, looking incongruously white and clean compared to its surroundings, and approached him.

XKD-47 tried to establish a datacast, but his systems were too weak to emit more than a garbled burst of static. Realising this, the new unit opened its mouth and struggled to pronounce unfamiliar syllables. “State… your… identity.”

“Stan will do,” replied XKD-47. “What is your purpose?”

The unit stared at him, the glow of a fusion reaction building in its chest. “To destroy… my enemy.” Its outer casing began to flow and distort from the tremendous heat that would, within moments, expand outwards and consume several miles of the unending alien architecture.

XKD-47 nodded. “I understand,” he replied, though it wasn’t really necessary. Of course he understood. They had all understood.

On the three-hundred-and-seventh day, XKD-47 completed his journey.