Sweat dripped from Clara's forehead and onto the dirt floor. As she crawled through the tight tunnel with Doctor Chang following right behind her, she slowly became aware of the unbearable heat radiating from the walls. She tugged her leather jacket off and leaned against the wall. Doctor Chang stopped and leaned against the earthy wall next to her, catching his breath.

"Are you sure this is the right way?" He asked hesitantly.

"You escaped from your first cell from here, didn't you?"

"Yeah, well…"

"Then this tunnel should be perfectly safe!"

Doctor Chang hesitated.

"I built a network of tunnels just a bit far down. They branch out, and it's almost like a maze, you know, in case -"

"In case anyone follows you. And you're the only one who knows the way out… Oh that's clever, that's extremely clever."

Doctor Chang smiled nervously and Clara felt the brief flutter of anticipation and fear.

"Lead on, Doctor Chang!"

The Doctor hit his sonic screwdriver in exasperation for the third time.

"Clara? Clara!" he shouted down the tunnel. His screwdriver whirred to life and he aimed it down the tunnel.

Missy rolled her eyes for the upteenth time in the last few minutes.

"Oh, don't even bother. She's too far gone in the tunnel for your screwdriver to find her. What are you going to do with a screwdriver anyways? Assemble a cabinet?" She scoffed.

"What is your pet project, anyways?" He demanded, eyes ablaze with fury.

Missy laughed. As she extended her Mary Poppins-esque umbrella to the Doctor, she quips "What do you know about the vipers of Valorsha?"

The Doctor went silent, his whole body tensing up with fear and apprehension.

"Why were you experimenting on them?" He said, darkly. "They're deadly snakes, making them deadlier by altering their DNA is just pointless."

"Well, clearly you haven't been paying attention to our Academy lessons, dear Doctor." She said snarkily, "They're deadly plant-snakes. Fascinating, really. And hyper-intelligent, to boot. No one in recorded history has ever survived an encounter with these snakes. I just merely wanted to study how it killed. Curiosity, really. Didn't you say it once? 'Science leads.'"

"You can't just release a bunch of deadly snakes in a prison!"

"Don't thrust your pitiful morality on me, Doctor, there's nothing you could do"

Missy paused. Then she grinned, cackling like a madwoman. "Did you know, that the soil on this planet echoes screams quite well?"

The tunnel was a maze of endless turns and twists and frankly, Clara was exhausted. Clara gasped and slowed her crawl, inwardly cursing years of intended workouts that had never happened.

"When will we get there?"

"In just a quarter of a mile," Doctor Chang, "We could stop for a while."

"No!" Clara shouted, by accident. Then she coughed, alarmed at how her voice sounded bouncing off the walls . "We have to continue. The guards probably already know we're gone. There's no turning back now."

Doctor Chang hesitated. Clara noticed the slight stiffening of his frame, and suspicion bloomed inside her.

"What is it?" She asked hesitantly, unsure of whether she wanted to know the answer. "What is it that you didn't tell me."

Doctor Chang nervously gulp and adjusted his clouded black-framed glasses. He opened his mouth to speak and seemed to decide against it.

"Come on, this way."

He began to crawl but Clara quickly grabs him by the coattails of his lab coat.

"Not so fast" She said. "What is it? What is it that you're not telling me."

"I-It's probably nothing." He replies. "Come on, this way."

"No, it's not nothing if you're thinking twice about telling me. What. Is. It." She demands stubbornly.

He sighs and relents. "Well if you must know. It was a while back, after I met you and the Doctor. A-After 3W, I mean."

Clara nods slowly. "After you got 'vaporised', right?"

"Yeah. Turns out the vaporisation didn't kill me. It was sort of a…sophisticated transmat device. It could beam things across space and time. Which is how I ended up here. But the problem is with my boss was that she- she needed me to help her in experiments."

Almost curiously, Clara asked, "What sort of experiments?"

"On creatures. Alien creatures. She locked me in my old cell and everyday had guards march me to a makeshift laboratory and spend the whole day there. Believe me, I love to experiment as much as any other scientist but the things she made me do-"

He paused and shuddered at the memory.

"I've always wanted to see if there was life in the universe but not like this."

"What do you mean?"

"She made me genetically-enhance these creatures… these plant-like snakes. It's the only way I could describe them. My boss called them 'vipers of Valorsha'. My task was to study how it killed by putting in 'bait', so to speak. No creature - alien or humanoid - survives and I doubt we will if it catches up to us."

"Is that why we can't go back in there? Those snakes, they're there?"

Doctor Chang sat opposite her and removed his glasses and began to wipe them clean on his dirtied shirt.

"Well, good luck with that," quipped Clara dryly and she mentally berated herself for being so Doctor-ish.

Doctor Chang laughed nervously and Clara noted that it was more of a nervous tic more than his glasses being dirty. An awkward moment passed between them. Above them, the clanking of chains and heavy footsteps were heard as the prison roared to life. Doctor Chang motioned for Clara to follow as he began to crawl down the dark tunnel once again. As Clara followed, a small sensation rippled through the back of her neck. The hairs at the back of her neck stood at full attention and she gave a small yelp of surprise.

Doctor Chang spun quickly to face her and cupped his hand around her mouth, silencing her. He put his fingers to his lips and paused. He peered over her shoulders before completely freezing, his face betraying his fear.