Bill and the Doctor had sworn not to leave the university. And they were serious. Yes, of course they were. They couldn't lie to Nardole, now could they? What kind of friends would they be? And why on earth would the Doctor leave his post and run off to some other planet when he had a vault to guard?

These thoughts glided through the time travellers' heads as they dashed down yet another corridor of the strange alien hospital they'd found, tucked in the back between 'oh my God was that a zombie?' and 'This place is abandoned and creepy but it's so, so cool'.

The Doctor turned sharply to the right and Bill had to catch herself on the corner wall to keep up with him. She could only hope he had any idea where he was going. Frankly, she was too scared to ask. That, and too busy scanning the halls for the zombie creatures.

Finally they reached a small inlet and the Doctor crouched behind a large dilapidated box of what used to be medical supplies. All of them were worn and rusty, left behind and forgotten for months, maybe even years.

"There must be a chemical of some kind, turning the people into…"

One of the creatures suddenly lurched towards them out of the darkness. Bill nearly screamed, "Zombies!"

The Doctor took her hand and they sprinted off into the corridors again, jumping over fallen, rickety chairs and narrowly avoided the shards of broken glass and unhinged doorframes in their path. Bill had a sudden, terrible thought. "Are we going to become like that?"

"Well, that really depends on what kind of chemical was used." Bill looked horrified. "If it's any consolation, we'd probably had felt the effects by now. So either the chemical is gone, or it only affects certain species."

The jogged the rest of the way down one corridor and stopped beside a stairway. The Doctor's eyes suddenly lit up. "Wait! Sonic! I can scan and see if there's anything in the air."

From his inside pocket, he pulled out his screwdriver, whirring it in the air in front of them. He frowned. "Nothing." He slipped the sonic back into his pocket and gazed into space. "What could it have been?"

Bill tugged on his arm suddenly, pulling him toward the door to the stairwell. "Doctor, they're coming!"

They hurried down the steps and out the basement door. The air was somewhat more misty down here. Ahead, a door was ajar, a white smoky substance filtering out. The Doctor stopped dead in his tracks. "Stupid, stupid, stupid."

Bill noticed her tutor wasn't beside her and backtracked a few steps. Some of the zombies were already descending the stairwell, their slow, ambling walk no less frightening than a bull's charge. "Doctor, come on! We have to...Doctor?"

The Doctor's right hand flew to his chest, eyes blinking hard. "Bill, I know what the chemical is. Don't worry. It only affects...gah!"

He dropped suddenly to a knee and Bill ran to his side. "Doctor!"

With gritted teeth, he ground out, "It only affects two-hearted species."

Bill's eyes went wide as she held onto the back of the Doctor's shoulders. "Are you going to become one of those things?"

He shook his head. "No...each species has a different reaction. It's just going to stop my hearts."

"Oh, there's a relief."

Bill's sarcasm died from her lips as the Doctor shuddered suddenly, letting out another pained gasp. "What can I do?"

She eased him to the floor with her as his eyes struggled to focus on her. "TARDIS...we need the TARDIS."

Bill looked up at the ceiling hopelessly. "That's three floors away!"

The Doctor sagged in her arms, his eyes rolling back. Bill smacked his cheek lightly and squeezed him tighter to herself. "No, no, no; stay with me. Stay with me, Doctor."

She placed two of her fingers on his neck. There was only one heartbeat. His eyes fluttered open again and danced across his face. "Bill." She leaned closer. "Think of the TARDIS. Close your eyes and think hard."

Tears sprang into Bill's eyes. Was this it? Was this how they went? Zombies coming in the door, in the basement of an abandoned, alien hospital? Bill rocked back and forth, holding the Doctor in her arms. "How can I think of the TARDIS right now?"

The Doctor widened his gaze up at his companion and found some extra energy. "Bill, I'm not just trying to comfort you. The TARDIS can hear your thoughts. You can bring us home."

Bill shut her eyes instantly, not wasting a second to chide herself for her misinterpretation, and pictured the Console room. Lights flicking on to greet her as she entered; Nardole wearing something odd he'd found in a thrift shop somewhere; the Doctor flying around the controls like he was a kid.

Suddenly the air changed, and Bill heard that unmistakable groan of the beautiful old time machine. The air cleared, and the moans of zombies disappeared. Bill opened her eyes and laughed at the sight of the Console panel sitting above she and the Doctor...the Doctor!

Bill looked down and found the Doctor's eyes closed. Her heart hammered in her chest, but her fingers found a weak, thready pulse in his skinny neck. His breath wheezed between pale lips. Bill combed a thumb through his hair as she got her thoughts in order. "We're home. We're okay."

Carefully, Bill laid the Doctor on the floor, rolling off her coat and setting it under his head as a pillow for comfort. She watched him for just a moment before dashing to the underside of the control panels and pulling out the medkit. It clunked to the floor as she haphazardly tore open the seal and pulled out an oxygen mask.

"Okay, Doctor, you saved me. Loads of times. Now it's my turn to save you."

With the oxygen mask on his face, he looked even more helpless than before, but his breathing did slightly improve. Bill chewed her lip as she looked him over, then felt her heart leap into her throat as a door downstairs banged shut. For a moment, the image of an alien zombie popped into her head. Then Nardole's voice filled the space.

"Don't think I didn't know you ran off again. Yeah, the TARDIS came to me first. She and I have a connection now, you see. Guess she doesn't want you getting into trouble, either. You really should...listen."

Nardole came to the top of the step and his eyes landed heavily on the Doctor. Bill's wide eyes watched him as she twiddled her hands together anxiously on her lap. Then Nardole sank to his knees beside them and all sense of authority left him. "What happened?"

Bill stammered the story out, pausing when Nardole drifted over to the medkit and pulled out a few strange medical devices Bill wasn't familiar with. When she concluded her tale, Nardole looked down at the Doctor, still shockingly pale and now breathing shallowly again. Quickly, Nardole reached into the Doctor's jacket and pulled out a stethoscope, which he placed over each side of the Doctor's chest. "His left heart's still not beating and the right's not sounding good either."

Bill took the Doctor's hand in her own and rubbed small circles on him with her thumb. Nardole picked up one of the alien medical tools and held it against the side of the Doctor's neck. "The TARDIS told me to give this to him."

Bill grit her teeth as Nardole pressed the trigger and whatever it was sank into the Doctor. Suddenly she felt him squeeze her hand tightly, his eyebrows furrowing. "Doctor?"

His eyes opened slowly and took in the two shales above him. His chest was still aching, but the toxins were finally relenting, succumbing to the antidote. He muttered something, but the mask and the slur in his voice made it impossible to hear. Nardole looked at the medical equipment again and picked up the one the TARDIS told him to, which looked like a sophisticated defibrillator. Suddenly he passed it to Bill, whose eyes went wide.

"I can't do this one. Part cyborg."

"Right." Bill let go of the Doctor's hand and stepped over him to sit on his left. His eyes peered up at her and she felt her hand shake so hard the device nearly dropped to the floor. "Right. Okay."

Nardole put a hand on her arm and gave her an encouraging smile. "He'll be fine. Won't you Doctor?"

The Timelord murmured something and gave Bill a warm, small smile. She tored her eyes to his chest, where Nardole was pointing to a specific spot. "Just put it right there, hard as you can, and press the trigger. It'll do the rest."

Bill readjusted her grip on the defibrillator and took a deep breath. 'Okay. Just saving the Doctor's life. Hopefully. Nothing big. It's all good.'

As soon as Nardole moved his hand, Bill plunged the device down and pressed the button, instantly feeling the Doctor jerk powerfully beneath her as a cry was wrenched from his throat. Bill's vision swam. Before she knew it, Nardole was prying the device out of her hands and rubbing her shoulder.

"It's fine. You did great; look! His hearts are both coming back to normal."

Bill caught her breath and she turned to the Doctor, whose eyes were looking blearily up at the ceiling. Nardole gave her a thumbs up and a smile as he lowered the stethoscope to his neck. "Good o'."

As the Doctor recuperated, Nardole and Bill took turns watching over him, taking short breaks to clean up and take a breath after all of the excitement and stress. After two hours of this cycle, Bill was sat by the Doctor's side, holding his hand tightly and watching his chest rise and fall with wonder. She couldn't bare to think about how close they had come to losing him. How dire the situation had been.

"Hey, ugh, Bill?"

She turned to Nardole, who was standing over her with two mugs of tea. He handed her the bigger one and she smiled in thanks. "I can take over, if you want."

She shook her head and took a sip of her tea, then sat it down as it burned her tongue. "Nah, that's okay."

Nardole sank beside her. "I'm sorry."

She eyed him with a smile. "For what?"

He looked almost pensive as he stared at a spot on the floor by the Doctor's legs. "All those times I shouted and swore something would happen if he didn't stay on Earth...I never really thought…"

Bill pulled her friend into a hug. "Hey; none of this is your fault or anything. We probably should have listened to you." She looked back at the Doctor, who was still fast asleep. His color had mostly returned but he still looked incredibly weak with that oxygen mask on his face. "Maybe he'll actually learn something from this."

Nardole chuckled. "I wish."

They shared a laugh, only stopping when the Doctor's head tilted and an indistinguishable mutter came from his covered mouth. Nardole helped him pull the mask off. "What was that, sir?"

The Timelord's eyes cracked open, bright and alert. "You should know by now that I don't learn very well. More of a teacher, me."

Bill furrowed her brow at him. "How'd you become a professor if you can't learn anything?"

"I talk and make myself sound important. How did you think it was done?"

The trio shared another laugh, this one at the expense of no one. The energy in the TARDIS was finally a happy one again, without any of the anxiety it had held earlier. Slowly, the Doctor's companions helped him sit up, then, much later, stand up. When Nardole left to brew up some more tea, the Doctor drifted to the Console and tapped in some coordinates, turning a dial or two as he steered. Bill put her hands in her back pockets and stared at her shoes awkwardly. Without looking, the Doctor asked suddenly, "What's wrong?"

She turned up to find him still staring at the screen. "Nothing. 'M fine."

He looked over at her, and she shuddered as she rememebered that helpless feeling; kneeling above him as his one heart beat weakly; holding the defribulator with shaking hands and tears in her eyes; the pain in his scream as she shocked his other heart back to life. She looked up and found his figure bigger and mistier than before.

"Doctor, you almost died today. I, I froze. And then I hurt you."

"Bill Potts, you saved my life."

She looked up at him and then her eyes drifted down to his chest. He pulled her close to him and she closed her eyes as his heart beat beside her ear. Silently, she smiled, then returned the hug. Nardole entered a few seconds later. "Aw, isn't this nice. Let me get in on this."

The Doctor groaned. "No, Nardole-"

Nardole wrapped them both in a bear hug, which the Doctor sank into without hesitation. Bill and Nardole smiled.