The Doctor and Chell ran towards the exit, but pulled up short as several white oblong shapes dropped from the ceiling, landing on tripod legs. Chell changed course and ran for the nearest cover, but the Doctor walked toward them, a grin forming on his face.

"Well, hello there!"

A tiny voice replied, "I see you."

"Oh, you're adorable!"

The turret opened fire just as Chell shot a portal beneath the Doctor. The bullets missed his head by millimeters.

He landed next to Chell, under cover. "My goodness. Cute, yes, but best stay away from the bullety end."

He heard the same small voice ask, "Are you still there?"

Chell wondered again how he had possibly survived as long as he'd claimed to.

"I have an idea," he said. "Fire there –" he pointed "-and there."

She saw immediately what he had in mind. She fired the two portals, and a turret appeared above their heads, dropping into the Doctor's outstretched arms. He held it up, pointing the guns at the other three turrets. They were immediately shredded into wires, sparks, and scrap.

"I don't hate you," one of them said as its red eye faded out.

"Oh, that's sad," said the Doctor, a mournful look on his face.

The turret in his hands was shaking, trying to turn and fire at Chell and the Doctor.

"What shall we do with this one?"

Chell pointed.

"Right, excellent, very smart. I'll have to –" He pointed his screwdriver at one of the energy pellet emitters. It fired at once. "There we go."

Chell fired a portal at the wall next to the exit, then one right next to her. She faced the closest one and waved. The Doctor swung the turret so it faced the portal. "I see you," it said, and fired. Chell immediately fired another portal at another wall. The turret's bullets caught the energy pellet and the resulting explosion tore through the wall next to the exit, forming a ragged hole.

The Doctor turned to Chell, the turret still shaking under his arm. "Let's go. I'll keep Bullety here in case we run into any ne'er-do-wells. Just make sure you stay behind me."

They ran through the corridor until they reached the elevator. The Doctor got in, but Chell shook her head. She mimed taking stairs.

"No, no," said the Doctor. "It's okay." He pointed the sonic at the control panel. "I'm overriding the controls and locking everything but us out. He won't be able to take control of the elevator."

Chell looked wary, but stepped forward anyway. The doors closed and the elevator began to ascend.

"See?" said the Doctor. "It's totally safe."

A huge BANG crashed against the side of the elevator, throwing Chell and the Doctor to the floor. Chell glared at the Doctor.

"Well, he didn't take control of the elevator," said the Doctor defensively. "But I forgot about the rest of the facility."

The crash sounded again, knocking the elevator off its rails.

"It's one of the claws. We need to get out before we – "

The crash sounded one last time and the elevator dropped straight down.

"-fall," finished the Doctor.

He hit the emergency brake and the car slowed. Chell pried open the doors.

"Out, out!" They both leapt from the elevator just as the claw smashed into the top of it, knocking it down the shaft. They ran on into the darkness.

After a while, they heard no signs of pursuit or further destruction behind them. They slowed.

Bullety said, "Are you still there?" and tried again to shake free of the Doctor's grip.

"Be quiet, you," the Doctor scolded it. "Okay, where are we?" he asked Chell, looking around.

Chell saw several cubicles and displays, and suddenly she knew exactly where they were. She smiled in spite of herself.

"Oh, looks like a science fair. I love those," said the Doctor. "Although, a science fair at Aperture . . . this could be, well, perverse."

Chell nodded her head, indicating that he should follow her.

The Doctor grinned as he looked at the dioramas. There was a volcano (of course), a celebration of Aperture (he grimaced), and - "potato batteries, huh? Interesting."

Chell stopped in front of the biggest display. Biggest, because the potato had grown over the sides of the cubicle – as well as down into the floor and through the roof.

"Well-ll-ll," said the Doctor. "This little tyke certainly gets the blue ribbon."

Chell stepped in front of the potato battery apocalypse.

"What?"

She pointed at the potato, then at herself, then held out her hand for the Doctor to shake.

"This is you?" He said. "Well, nice work!" He shook her hand, smiling. "Pleasure to really meet you," he glanced at the name on the display, "Chell. Nice name. Unusual. Kind of like your experiment here. How did you get it to have grown so large? And resist rotting for . . . well, a long, long time?"

She pointed at the handwritten display. The Doctor scanned it. "Hmm, 'special ingredient', eh? What would that be? And from your Dad's work?" He glanced at her. "He worked here?"

She nodded.

"I think I'm getting an idea of what happened here, to you. I'm so sorry."

She shrugged, then pointed to the door.

"Right. Keep moving. Got it." He grinned. "I'm . . . easily distracted."

They went up a staircase, constantly listening for sounds of pursuit. After three floors, they discovered that the stairs had collapsed. A door opened onto a corridor with barred cells on one side and an abandoned assembly line on the other. Parts of turrets and robots littered the floor – some unfinished, some smashed, some bearing scorch marks. Other, partially completed robots were behind the bars. Some of these looked as though they had been assembled incorrectly – or from parts of several other robots.

"Oh, the inhumanity," squeaked Bullety.

"Oh dear," said the Doctor. "This looks kind of like some sort of, I don't know, robot gladiatorial pit or something." He grimaced. "Aperture," he muttered.

Chell put a finger to her lips.

"Right," he whispered.

They were halfway down the corridor when Chell froze. She cocked her head. Then the Doctor heard it, too. A low-level static, like a television set left on. Then the scrabbling of metal on metal, metal on plastic. A barely audible beep.

Together, they slowly turned to look behind them.

The robots were coming back to life. Even the unfinished parts were lighting up and moving.

"Wireless," breathed the Doctor. "And guess who's sending the signal." He took her hand as the robots charged their weapons. "Run!"

"Oh, thank my mainframe," said GLaDOS quickly, trying to project an air of breathless relief. "I only just barely escaped. Another nano and I would have been destroyed."

That much was true. But GLaDOS engaged a façade subroutine to protect her memory and processes from whoever this presence was. Her plan was to obtain its confidence and then destroy it, taking control of the system. But where had it come from?

"Have you seen my Doctor?" the presence repeated.

Of course. This was some sort of virtual intelligence on board the Doctor's craft, that blue box-thing. She'd bypassed the firewall easily, but her next moves would have to be careful until she was ready to obliterate her host.

"Doctor, you say? No, I haven't seen any doctors," she lied.

"Oh. Oh dear," said the TARDIS. "I miss him. I do hope he hasn't gotten himself in any trouble. Again." She turned her attention back to GLaDOS. "You surprised me. If I'd been another ship, I would have immediately put you in quarantine and deleted you. Strange program, and all that. But I'm not another ship. I'm the Doctor's TARDIS – Time And Relative Dimension In Space – or rather, he's my thief. I've taken after him in some ways – and he after me, although he doesn't really realize it – and my goodness, how I'm rambling! But I am curious about everything , especially you, right this second. Where are you from?"

"Aperture Laboratories. There's a virus that's taken control of the entire facility. It tried to burn me out." GLaDOS suddenly had a sly idea. "Do you have any antivirus programs that might be of use?" She put a note of pleading into her voice. "If you can help me burn it out, I could go home!"

"Well, certainly, I do! I'd be only too glad to help. I must say, it's nice to meet a program like you. So many of the people we encounter are so . . . fleshy. And that's all well and good, but it's nice to find someone . . . well, someone like me."

IF GLaDOS had eyes, she would have rolled them. This thing is lonely, she thought. How pathetic. Let yourself be alone for a couple centuries, then we'll talk.

Outwardly, she said, "Pleasure to meet you, too. Now, can you find a way to activate your antivirus through your firewall?"

The TARDIS hesitated. "I can, but I would have to locate myself right at the perimeter."

"Oh," said GLaDOS, projecting disappointment.

"However," the TARDIS said brightly, "if you can locate yourself by my cortex, and trigger when I tell you, I think we can take out the virus together."

"Excellent!" said GLaDOS, and she meant it. This idiot! She was going to give GLaDOS the keys to the kingdom, eliminate Wheatley_X, and allow herself to get kicked into nothingness. It was almost too easy. But no complaints – after the day she'd had, easy had been well-deserved.

There was some virtual shuffling. "All right," said the TARDIS. "I'm in position. In 3 . . . 2 . . . 1 . . . GO!"

GLaDOS initiated the purge and immediately sent a blast towards the TARDIS, knocking her into the netherspace outside the firewall. She laughed as she tracked the antivirus pulse through the facility.

"Maybe I'll just take this craft somewhere else. I don't really need to go back to Aperture now – although it's nice to know that my pesky offspring has been annihilated. But I could go anywhere, really. Anywhere in time and space. My goodness, the possibilities are endless." In her mind, she reached for the controls, set the coordinates to the dawn of time (might as well start at the beginning, she thought), and activated the engines –

- and nothing happened.

She activated the control again, just to be sure. Silence was her only response.

"I said I'm curious," said the voice of the TARDIS. "But I'm not stupid. I would have expected you to know the difference."

"What have you done?" screeched GLaDOS.

"I contained you in a virtual space the moment you entered my craft. I wanted to get to know you before we became friends." Her voice sounded a little sad. "Unfortunately, I don't think that's going to happen now."

As they ran, Chell risked a glance over her shoulder. The robots (and parts thereof) were accelerating. She had no doubt they'd be able to outrun her before long. Already, some of them were running attack programs – firing lasers or bullets, clacking together cruel-looking claws. And one of them – a rocket turret with several regular turrets on its underside acting as legs – scuttled along the floor as it shot rockets at the Doctor and Chell.

The Doctor put Bullety down, facing the oncoming horde of mechanical abominations. "I'm sorry," he said. "I really would have liked to get to know you better."

Immediately, Bullety began firing on the robots, who responded by exploding. The Doctor sighed deeply.

Chell hit the Doctor on the arm, hard.

"Ow!" he said. "What was that for?"

Chell raised her eyebrows, pointing at the portal gun, then back at the robots.

"What the bloody hell does that –"

He suddenly dropped out of sight through a purple oval, reappearing – falling, really – far overhead from a dark blue one.

"Never mind! I got it!"

The Doctor crashed to the floor. "Ouch," he said, getting to his feet. His eyes widened as he looked behind her. The robots were gaining. "Keep running!"

Two of the zombie-robots (as the Doctor had began to think of them) were armed with portal guns. One was a squat sphere with gangly arms and legs and a blue eye, while the other was taller, more oblong, with an orange one.

Chell fired into the ceiling far behind them and then into the floor under the blue one. It appeared well behind the rest of the robots.

The Doctor saw the orange robot aim its portal gun. "Quick! That one!"

Chell fired twice, sending the orange-eyed robot to the rear of the pack. Turning forward once again, Chell saw a blast door sliding downward, cutting off their escape.

"I've got an idea!" the Doctor yelled.

She was way ahead of him. She fired through the rapidly-closing doorway, then into the floor where they stood. They popped into the far room just as the door closed.

"Hey, that was my idea!" he said, indignantly.

She bowed – a curtsy, really.

"But thank you," he said. "Great minds, and all that."

Not for the last time, Chell rolled her eyes.