The Doctor hated failure.
He kept his dark eyes set on the compact alien scanner that he carried with him, scrutinising the simple wire frame graphic representing the vast city and silently willing the device to reacquire the biological signature that it'd been tracking only moments ago.
The scanner's blinking status lights finally turned red. The distinctive signature, different from any other to be found on this world, had vanished.
His prey had eluded him again.
He drew in a long breath of air past his thin lips, tasting the various toxins present in the atmosphere as he inhaled. Grimly, he realised that this was perhaps the most polluted place he'd visited in many years, although the horrific slave-mines deep within the crystalline-mantle of Xenithar Four came close.
Grimly accepting his latest failure, the Doctor switched off the scanner and slipped it back into the pocket of his leather jacket, still wet from the latest shower. The scanner itself was a basic yet effective piece of technology that he'd once borrowed from a man on Vakinthi Prime's third moon over fifty-thousand light-years away. In the five lifetimes he'd lived since borrowing it, the Doctor had never gotten around to returning it.
He stood in a rain-soaked alleyway that ran between two dilapidated buildings, buildings whose neglected appearance was mirrored in a hundred others throughout the docklands. What streetlights still worked still shone their dirty light down onto the litter-strewn roads and pavements, casting unnatural looking shadows onto the crumbling brickwork.
In his other pocket, the Doctor felt the gentle vibration of the mobile phone Rose Tyler had insisted he brought with him. He reached in and withdrew the phone, pausing only briefly to check that he had the right key before pressing it and holding the phone to his ear.
"Hello?"
"It's just gone five am, Doc," Jack Harkness told him curtly. The fatigue the other man felt was audible in his voice, obviously the result of having gone almost without sleep for the three days since the Tardis had arrived on this world.
The Doctor closed his eyes, pressing the fingers of his free hand to the bridge of his nose. He himself had managed to steal a few hours of restless sleep earlier that day, after working with Jack and Rose to set up the network of motion-sensors throughout the docklands ready for tonight's fruitless search.
His headache had begun as he slept, and had subsided before reasserting itself with a vengeance in the last hour.
"You there, Doc?" Jack asked, obviously growing impatient.
The Doctor knew that reminding him of the time was Jack's way of telling him that it was time to quit, for them all to return to the Tardis to rest and reassess the situation.
Maybe he was right.
Another sound reached the Doctor's ear, a long, drawn-out yawn transmitted by the other mobile phone that was also part of the three-way conversation.
"You still with us, Rose?" the Doctor asked, turning on his heel and beginning his long walk back down the lonely alleyway that he'd come running into only moments earlier, following what he thought was a positive-contact by his scanner.
"Where else would I be?" the young woman answered, managing to sound playful in spite of her tiredness.
"We should all get some shut-eye," Jack stated flatly, persistently. "We're never gonna find this thing if we're all half-asleep."
The Doctor nodded, silently acknowledging the man's point. "Meet me back at the Tardis," he instructed them both.
Unwilling to be drawn into a conversation, he pressed the key to end the call without saying anything further.
Alone in the darkness again, the Doctor privately scolded himself for not signing off properly, for not saying goodbye. Yes, he had come to depend on the two human companions who accompanied him through time and space, but his feelings for them ran deeper than mere dependency.
He had come to enjoy the company of Rose and Jack, of Rose in particular, and knew that they deserved better than the brusque manner in which he'd just spoken to them.
But after spending three days tracking the creature across the vastness of New York city, culminating in yet another failure, there was no conversation left in him.
The Doctor looked up past the rusting fire-escape ladders that clung to the side of the buildings and into the night sky, at the millions of distant stars that even he couldn't recognise.
How many of those sparkling points of light were orbited by planets that he'd visited over the course of nine lifetimes? How many were home to cultures that had been influenced by his presence as he travelled through the galaxy's past, present and future?
And why in a universe of so many incredible civilisations, was he inevitably drawn back to this one? An unremarkable planet, inhabited by a people equally as unremarkable as the world where they'd evolved.
Earth.
"How many times have I saved you?" the Doctor said aloud, his quiet words echoing in the silence of the alleyway.
As he'd expected, the planet offered no answer.
He strode quickly out of the alley and onto a wide road devoid of people and cars, listening to the sound of his footsteps on the wet pavement. The thunderclouds that loomed overhead told him that yet another downpour was, as Jack would say, "on the cards."
But the coming shower was not of particular importance to the Doctor, for he had a far more pressing concern. One that had been with him ever since he'd parted-company with the others earlier that night and begun searching the city for his quarry.
When he'd begun, the city had been bustling with people enjoying the local nightlife, but as the dawn approached the streets had cleared of inebriated humans, leaving the Doctor to wander through the darkness unhindered by the people who would never, could never know of his presence.
And as he lurked in the shadows, becoming little more than a shadow himself, he knew that he was not alone.
He was being followed.