It was a Mark VI combat mechanical and it had been hunting them since daybreak. The machine had pursued them through the Forest of Desolation, across the Burning Grounds, into the Valley of Agonies, and to this place. Now the chase was all but over.
The Mark VI adjusted its stealth settings and edged closer to the cave. Inside, the four humans had chosen to make their last stand. During the long pursuit, the machine had assessed each of their capabilities, finding none to be a match for its own.
It ran through their profiles now.
First, the younger male human, designation Ryan. Minimal offensive capabilities. Unarmoured. Unlike the Mark VI’sactivephlebotinum armour, his cellulose-based clothing provided no effective protection. Similarly, although his rubber-and-plastic shoes offered traction, they were no match for its military-grade tank tracks. The machine accessed the next profile: the younger female human, designation Yaz. Her actions during the pursuit had demonstrated evidence of military or law-enforcement training, however it was clear that she lacked battle experience. The Mark VI, meanwhile, had seen action across the known galaxy in the service of  the Fleet before  it  had been drafted to  the Citadel. She posed no threat.
Next, the older male human, designation Graham. Eyesight fading, bone density weak, hairline receding. He would be crushed as easily as a bugbeast of Zeta Draconis. It took a microsecond to dismiss him.
And, finally, the other female human. The Mark VI had obtained little intelligence about her during the pursuit besides the fact that she was clearly the group’s leader.
Before launching the assault that would inevitably lead to their capture, the Mark VI took a moment to scan the humans one final time. Its finely tuned audio-detection circuits picked up a snatch of conversation.
‘I wouldn’t call it a giant robot,’ said the female leader. ‘Don’t exaggerate, Graham.’
‘Well, excuse me, Doc. I didn’t know there was a minimum height requirement.’
The Mark VI analysed Graham’s voice. The waveform suggested anxiety, along with the characteristic known as sarcasm.
‘All I know,’ he went on, ‘is that we’re being chased by a bloomin’ great metal monster.’
‘Actually, it’s not metal,’ said the woman the Mark VI now knew to be designated Doc. ‘The shell is some kind of composite material. Ray- shielded, but not invulnerable. Anyway, I’ve seen everything I need to. That’s enough running around for one day.’
‘What are you talking about?’ said Graham. ‘You planned this, didn’t you?’ said Ryan.  ‘I
knew it!’
‘You mean I didn’t have to knacker myself running all over this planet?’ Graham protested. ‘How else was I supposed to figure out what we were up against?’ said Doc. ‘Had to give our
friend a proper run-out.’
The Mark VI paused. Its audio scan had detected something unexpected.
Ba‑doom. Ba‑doom.
Overlapping heartbeats, emanating from Doc. The machine drew the only logical conclusion: she had two hearts. It made the necessary correction to the profiles. Three humans. One unknown.
Identify.
The Mark VI connected to the Fleet network, and sent its query winging back to the Citadel’s supercomputers, accessing the knowledge of 10,000 star systems. Four milliseconds later, it had an answer.
Species: Time Lord. Origin: Gallifrey.
Designation: The Doctor.
A list of the most dangerous species in the universe scrolled across the Mark VI’s display: Sontarans, Cybermen, the Daleks of Skaro. According to the database, they had all fallen to this Doctor’s sword. Not that the machine could actually identify a sword – or indeed any kind of weapon at all – on her person.
The Mark VI hesitated.
Until that moment, every factor had pointed to an overwhelming victory in its favour, but this new information prompted it to inject a note of caution into its plan. Rather than risk close combat with a being so obviously lethal, the machine adjusted its tactics, choosing instead to launch a ranged attack. Selecting its primary armament – a plasma-beam blaster built into its right arm – the Mark VI activated the laser rangefinder and calculated a firing solution.
 
‘No good,’ said Yaz, returning from the back of the cave. ‘It’s a dead end.’ She pointed to the cave’s mouth. ‘That’s our only way out.’
‘Are you sure that robot thing’s even still out there?’ asked Ryan, squinting into the fast-fading daylight. ‘Maybe we gave it the slip.’
With a noise like a great gulp, the interior of the cave suddenly bloomed the colour of storm- light. The air sizzled and a burning smell filled the small space, as Ryan scrambled frantically towards the back of the cave. Looking over his shoulder, he saw the boulder he had been sheltering behind was split cleanly down the middle, as neatly sliced as a loaf of bread.
‘And I suppose that isn’t a death ray,’ grumbled Graham, brushing stone fragments out of his hair.
‘Oh, that’s definitely a death ray.’ The Doctor grinned.
Ryan and Yaz shared a look.
‘I’ve seen that grin before,’ Ryan whispered. ‘On Proxima Ceti, just before she outwitted those  carnivorous chessmen.’ Yaz nodded. ‘And on that derelict space station, when she worked out how to defuse the temporal anomaly bomb with three seconds left on the countdown.’ ‘Couldn’t have done it without you,’ the Doctor said, striding past them.
Ryan hadn’t travelled with the Doctor for long, but even in their short time together he had seen her make impossible escapes more times than Harry Houdini – and one time he’d even seen her escape from Houdini. Well, not the real Houdini, but 200 evil cyborg clones of the great escapologist in the subways of New York City in 1904. But that was another story. What, he wondered as he watched her march towards the cave entrance, could she possibly have planned now?
The Doctor stopped beneath the stone arch at the cave’s mouth. This planet had a short day– night cycle, and the sun was already low on the horizon. The last of its slanting rays cast a golden aura around the Doctor’s silhouette. Standing there, glowing, she looked invincible – perhaps  even immortal.
She raised both hands.
‘We surrender!’ she called out.