Light streamed through the darkened room, as The Doctor slowly regained consciousness. She'd been slipping in and out for a while. Right now, she was just focusing on trying to stay awake. If she could stay awake, then she had a better chance of getting out of here alive. God, her head hurt.

There wasn't much to observe. The small beam of light only occurred when one of her

captors entered the room. Presumably to make sure she was still alive.

So they hadn't killed her instantly. That was good. That meant they needed her for something. But for what?

Out of habit, The Doctor tried to move, but found that her arms were held in place by chains. She was being held up by the metal chains, which explained the ache in arms. Her feet were strapped to the floor in metal vices. Looked like she wouldn't be getting out here easily.

Groggily, whilst she tried to break free, searching for a weakness in the chains, that's when she felt a sharp pain shoot up her arm. Glancing to her left, to her horror, she saw tubes coming out of her arms. Fear shot through her, as she realised the tubes were blood red.

She turned back to her captor, who was coming towards her at a menacing rate…

…

'Yaz? It's Martha. I'm in Sheffield now, where shall we meet?'

'Come to the TARDIS, it's probably the best place to talk.'

…

Yaz took in the beautiful stranger. She wasn't sure what she was expecting, but she hadn't imagined Martha to be so… well, stunning. Yaz wondered briefly at the nature of Martha and The Doctor's previous relationship, but batted it away as soon as it arrived. Now was not the time to get jealous, Khan.

She watched as Martha drank in the TARDIS, smiling so widely, as if she was greeting an old friend.

'Hello, old girl,' she whispered. 'It's been quite a while.' The TARDIS responded, the central column humming contentedly. Martha definitely belonged here.

Graham had finally got away from the police officers, and had joined the youngsters in the TARDIS to greet Martha. The greetings had been jovial and professional, but now they had to get to work. The personal questions would have to wait.

'So, Martha, Yaz filled us in on how you can help us find The Doc, which is great. But what do we do now?'

Martha turned away from the console, back to the matter at hand.

'It's Graham, right?' She asked, wanting to get names right straight away. He nodded in the affirmative. 'Well, first thing's first, is we need to get a trace on her. What we need to do is find something of her's to analyse, and then depending on what we can use, use the corresponding tech we have at our disposal to find her.'

'Wow, is it really that easy?' Ryan asked, finally feeling optimistic. Martha let out a nervous laugh.

'Well, yes, in theory. However, it can take a while to work. We scavenge alien tech, but often have to patch it up, which can take time.'

'What can we scan of The Doc's?' Graham asked.

'It could be almost anything, but the best matches would be something biological, like hair, DNA, blood…'

'Can't we just use the blood found at the scene?' Yaz asked, her stomach turning at the thought that she had just sounded so cavalier, given that it was The Doctor's blood she was talking about.

'There's gotta be something easier, something right here in the TARDIS? A hair from a hairbrush, maybe?' Graham suggested. Surely they didn't have to go to all that trouble for something they could already use?

Unfortunately, Martha shook her head.

'There's not enough DNA for the tracer. Yes, Yaz, the blood is a better way to go. Trouble is that it's all been cleared up now, so we're going to have to do a bit of 'breaking and entering' to get it.'

Yaz had a feeling that Martha was used to 'breaking and entering.'

'How do we do that?' Ryan asked, feeling a bit uneasy. Martha looked straight at Yaz.

'Yaz, how do you feel about going undercover?' The thought filled Yaz with excitement. She couldn't deny it. She'd always wanted to go into covert forces further into her career.

'Anything for The Doctor,' she answered honestly. She blushed as soon as she'd said it, but no one seemed to notice. Martha smiled at her, knowing Yaz was just like her.

'What about Ryan and I?' Graham insisted, not wanting to be left out of the plan.

'So, Yaz and I will go undercover to retrieve the blood. My husband is coming up to Sheffield with some tech that will help us. He'll need your help to get it working, and we're going to need your help to get us in and out of the base. Have you got a car?' Graham nodded. Martha turned to Yaz.

'Yaz, the blood will have been taken back to your station. I need you to get me in.'

'But how? I'm just a PC. I don't have any power.'

'Don't worry, we'll think of something. Let's brainstorm.'

…

After an hour or so, the team of four had a plan.

Yaz would get into uniform, and head back to her headquarters first. It would arouse less suspicion if she and Martha didn't arrive together. It would allow Yaz to survey the scene. She would then send Martha a message, letting her know when was best to arrive.

Martha would swan in, claiming to be the Head Medical Examiner of the Police Commissioning Board, and she had to examine the evidence. If she was to be questioned, which would be likely, she would just flash her psychic paper, which should get her through.

'Hang on, The Doc's got psychic paper!'

Martha chuckled.

'Where do you think I got mine from?' She replied cheekily.

This would give her the highest clearance. She would weave the government into the narrative and put on her best pompous voice. Martha had got pretty good at acting over the years.

Once she retrieved the sample, she would get out of there and head out back. Graham would then be there to pick her up, and take her back to the TARDIS. Yaz would then follow afterwards.

'Simple plan. Think it will work?' Ryan asked anxiously, aware of how badly this could go wrong. Particularly for Yaz, who would be putting her career on the line.

'It's got to work. We've got to get that blood,' Yaz answered resolutely.

So it was settled. Let the plan commence.

…

Yaz slipped into her uniform in the locker room, her heart beating erratically. She'd managed to evade her colleagues for now, as she headed to her desk to prepare for another shift. A shift that would be like no other.

She was breaking her own code of conduct for The Doctor. But then, when hadn't she done that before? And besides, The Doctor was far more important than her job. Yaz realised that no one had ever made her feel that; not even Danny.

She prayed that The Doctor would be easy to trace. This blood enquiry was their leading trail, and she didn't know what they would do if it turned out to be a dead end. She took a deep breath, and sat at her desk, trying to look as normal as possible.

She needed to get Martha into forensics. That's where the evidence would be held. She kept her personal phone hidden in her pocket. Normally she had to lock it away, but today, she needed to use it to alert Martha, so she could make her entrance.

Yaz tried to focus on the screen in front of her, but her mind was elsewhere. This had to work. She had to get The Doctor back.

Fear struck her then, as she agonised over where The Doctor must be now. She had to be safe, she just had to be, Yaz prayed.

She just wanted to get this over with, but undercover work required patience. She had to wait for her supervisor to arrive, before Martha could be alerted. The plan was that Martha was to ask for Yaz's supervisor. She would then butter him up and then ask if Yaz could show her to forensics. Yaz knew her supervisor would fall for it, given that he had when The Doctor had waltzed into the station. And besides, surely he would want to show off his best junior officer?

As if on cue, Ramesh walked into the station, making his way to his office. Yaz called out a greeting and he grunted back in recognition. Okay so he wasn't in the best mood. Great.

Still, it wasn't anything Yaz couldn't handle.

She knew that with coffee, he would perk up and behave more like a human being. She waited 15 minutes whilst he followed his usual start-of-shift routine, and then sent a surreptitious text to Martha.

Show time.

…

'And, why did you want to see me?' Ramesh asked Martha, confusion clear as day on his face. Martha had breezed in, playing her part. She had changed her name to 'Samantha Jones,' which Yaz had to make sure she kept in mind. Yaz stayed at her desk, trying to look as disinterested as possible. In reality, she was attempting to listen to everything being said. She waited with baited breath, her anxiety rising by the second.

To her frustration, Ramesh closed his door just as Martha started responding. Great. Now she had to rely on Martha's persuasiveness for the plan to work. Not that Yaz doubted Martha. No way.

She must have been good if she was travelling with The Doctor. And as Martha was willing to risk everything to get The Doctor back, Yaz knew that she must have been a true friend. However, there was something niggling at the back of her mind. She wouldn't call it doubt, rather, but curiosity. Martha had said she hadn't heard from The Doctor in 10 years. So what caused them to part ways? Why had Martha given up the travelling?

Yaz couldn't get that question out of her head. She couldn't ever imagine walking away from The Doctor. Ever.

According to plan, Ramesh opened his door, and called out to Yaz.

'PC Khan, can you come in here for a sec?'

Yes, she was in!

And now for the hard part.

…

Martha and Yaz had agreed that they would keep up the charade of being strangers, even when there was no one around. It was a police station; anyone could be watching or listening.

Yaz gave Martha a 'standard tour of the station,' as per Ramesh's request. Whatever Martha had said to him had clearly worked, because his gloomy demeanour from this morning had instantly changed after their discussion. He virtually waved them off, beaming.

Martha did, however, not let the fact of being undercover stop her from asking Yaz questions. She genuinely wanted to know more about the younger companion.

'So, PC Khan…'

'Oh, please, call me Yaz.'

'Sure, Yaz. So how long have you been in training?'

'About 18 months now. Just got 6 months left and then I become a proper officer, as long as I pass probation.'

'And do you think you will?'

Yaz had to admit, she was surprised by this line of enquiry. It seemed a little personal, considering Martha was supposed to be 'Samantha.'

'I think so. Hope so. I keep getting seconded though, is the only thing that might stop me. Ramesh has been at me about it, actually. I need to stay here to finish my probation, but then I get swept away…' she paused, realising she was being too truthful. 'Swept away by covert operations. But if I told you, I'd have to kill you,' she joked, not sure if her bad James Bond joke was coming off well. Martha nodded in response, suddenly looking wistful.

'When I was doing my medical training, I got seconded too. Took me longer to complete my training than I'd anticipated.' Suddenly, this was Martha talking, and not Samantha. Yaz stopped Martha walking, trying to understand the hidden code. What was Martha trying to tell her? 'Just, make sure covert ops doesn't become your life, yeah? There's more to life than that,' Martha said quietly, now looking at Yaz directly.

Yaz gulped, nerves setting in.

'Is there?' She responded, now fully understanding. Martha wasn't talking about covert ops at all.

'Secondments can be dangerous. Just, keep safe.'

Yaz was about to counter that statement, when Martha suddenly changed tack and transformed back into Samantha. 'So, which way to forensics?' She asked, suddenly increasing the pitch of her voice to match her character. Getting whiplash from the serious conversation they'd just shared, Yaz tried to catch up.

'Erm, yeah, right this way.'

…

The next stage of the plan was in motion. They reached the forensics vault, which was guarded by one of Yaz's colleagues, and friends, Will Huntley. He was a Forensics Coordinator, so their paths didn't cross much, but they occasionally had lunch together. As she approached him, the knot tightened in her stomach. Guilt tied itself in there with her anxiety, as she realised she wasn't just risking her job; she was risking his too.

'Hey Will, how are you?'

He smiled up at her brightly, and the knot tightened.

'Alright Yaz, how are you? Haven't seen you around much!'

'Yeah, well I got seconded…'

'Again? Man, covert ops must like you! Hey, next time you see them, could you, I dunno, put in a good word for me? Forensics is good and all, but a change of scene wouldn't hurt…'

'I'll see what I can do, Will.' If she was actually being seconded, she totally would have put in a good word for him. However, given that he wasn't aware of space travel, she figured The Doctor might not go for it.

Oh yeah, The Doctor. Focus, Khan.

'Hey, listen Will. This is Samantha.' She gestured to Martha, who raised her hand in a half wave. 'She's a high ranking Medical Examiner and I'm giving her a tour of the place. Could ya let us, so she can get more of a feel for it?' Will looked a little concerned.

'Yaz, you know we can't just let people in there…'

'Hi, Will, is it? I'm working on a very high profile case and I need to have a quick look at some of your records. Here are my credentials.' Martha held out the psychic paper to him. Will's eyes narrowed in concentration. He finished reading, and seemed slightly more appeased.

'Of course, apologies, Dr Jones. Go right on in.'

Martha pulled back to the psychic paper, and smiled at him sweetly. Got to keep on his good side.

'Thanks, Will. I'll put in a good word for you with Ramesh.' Will seemed to perk up at that, as he let them into the evidence room. Yaz followed Martha, the knot now pulling even tighter. She hoped Will would get off easily.

…

Of all the rooms in the station, Yaz knew the evidence room would have the most surveillance. Cameras, microphones, the works. They had to be super careful now. Yaz wasn't quite sure how they were going to get the evidence out of here, but she trusted Martha. She knew what she was doing.

Yaz had to give the impression that she was still giving a tour, and Martha had to look as if she was only taking a mild interest in the evidence samples that surrounded them. The reality was that they were both frantically searching for the The Doctor's blood samples.

'So, tell me, Yaz, where are the blood samples kept?'

'Well, it depends on the case. But generally, they are kept in the storage cabinet.'

'Which one?'

Yaz quickly scanned the room, and felt relief as her eyes settled on the correct cabinet. Bingo.

'The one right behind you. Since blood is one of the most commonly examined pieces of evidence, it's important that it's easily accessible.'

'Yes, I can see how that would be useful. Could I take a look?'

'Sure.' Yaz moved to stand next to Martha, and opened up the evidence cabinet. Inside were labelled tubes, vials of blood from various crime scenes. Yaz looked up at Martha quickly, trying to say with her eyes what she couldn't ask out loud.

How the hell were they going to find it in here?

Luckily, Martha knew her way around an evidence locker.

'So, evidence tends to stored chronologically. The most recent evidence samples will be stored at the front, so…' They both glanced down at the blood samples at the front of the cabinet. That had to be The Doctor's, right? It had today's date labelled on them, after all. The Doctor had only been missing for a few hours. Martha glanced back at her, and winked.

Yes, they'd found it!

'Ah, I see. You must have seen a lot of these then, being a Medical Examiner and all?' Yaz asked, now stalling for time. She hoped Martha would let her in on the plan now, as from here it was all hazy to Yaz.

Martha glanced briefly at the cameras around the room, as if she was assessing them.

'Yeah, yeah, something like that.' She sounded distracted, as her smile suddenly disappeared. Oh, no, that can't be good. Figuring Martha needed more time, Yaz tried to continue the conversation.

'Where did you study medicine?' She asked, saying the first thing that came into her head. Martha continued to survey the room, a frown now setting in on her lovely face. She grimaced.

'Oh Yaz, I am so sorry.'

Okay, what?

'Why?' Yaz asked cautiously, with the true question flashing in her eyes. Martha caught her eye at last, and Yaz did not like what she saw there. She saw determination, worry, and an apology.

Martha reached into her pocket subtly, and Yaz noticed, but didn't take note of it properly. Not until it was too late.

'Please forgive me,' Martha pleaded with her, as she quickly pulled something out of her medical coat. Yaz didn't have enough time to register it before she felt a sharp jab in her neck.

Martha plunged the syringe into Yaz's neck, and wrenched it away. Yaz immediately cupped her neck, feeling some cool liquid trailing down her skin. Yaz's eyes widened in alarm, too confused to fight back. What the hell was happening?

'What did you do?' She asked anxiously. This surely hadn't been part of the plan?

'I'm so sorry Yaz,' was all Martha could say, as she started to gather up the blood samples.

Yaz couldn't move. Her legs gave way underneath her, her vision blurring. She kept her hand on her neck, as she fell to the floor. Her limbs began to lose their stability, their independence. She desperately fought to keep her eyes open, but she was failing miserably.

Had Martha just poisoned her?

Terror, rage and extreme hurt came over Yaz, but it wasn't enough to overcome her now paralysed body. Martha looked back at her, before running out of the evidence locker. That was the last thing Yaz saw, before her world turned black.