Yaz was not having a great day.

First, Dan had left the TARDIS and, while Yaz did miss him already, she didn't remotely blame him for leaving. Given that he'd almost had his space helmet punctured, he probably deserved a bit of a quiet life. Like he said, the life Yaz had with the Doctor wasn't really for him.

Still, she missed him nonetheless. He had been there for her through those long years in the early twentieth century, making sure she didn't lose faith and keeping the mood light with a joke when things got too heavy. He had almost become like an uncle for Yaz; hopefully, they'd be able to keep in touch.

Then, the whole plot with the Master had happened, and Yaz had been thrust into the UNIT headquarters. And met Tegan Jovanka and Ace McShane. That had been a bit of a shock, to say the least. She'd met Captain Jack before, of course, but this was… different. Two women older than Yaz's mum had been travelling with the Doctor when they had been Yaz's age. That… shocked her.

Although she could guess that Tegan's "we used to be you" hadn't been referring to… whatever was going on between Yaz and the Doctor.

That was a thing now. Her and the Doctor.

Talk about a quick turnaround. All that business with the sea devils had drastically changed things between them. The Doctor did care for Yaz in the same way that Yaz did for her, but… the two of them couldn't do anything. Time was apparently running out for the Doctor. And Yaz wasn't sure she could bare to see the Doctor she loved change into a whole new person.

So, here they were. Trying to enjoy things while they lasted.

Anyway, back on topic.

The Master had turned up, caused all manner of difficulties. Now, Yaz was holding the Master under armed guard while the Doctor piloted them back to the Russian Empire during the First World War.

The TARDIS slowly stopped groaning, and the Doctor headed out of the doors, following by the Master, who was smirking in that infuriating way. Judging by Tegan and Ace's reactions to him, he had been causing trouble for decades. Yaz didn't want to know about the cat thing, though.

Yaz followed them, pointing the gun at the back of the Master's head. She was just reaching the doors when the room beyond them opened up before her-

Daleks and Cybermen. Dozens of them.

'Yaz?' the Doctor said, as she stepped out of the doors.

'Yeah?'

'…. I'm sorry.'

'What for-'

But Yaz heard the tell-tale sound of the sonic screwdriver, and the TARDIS doors slammed shut between Yaz and the Doctor.

'No!' Yaz screamed, charging forward. The gun fell to the ground, forgotten. Yaz slammed her hands against the blue doors, but she could already tell that they had been locked. Behind her, she could hear the TARDIS dematerialising.

Turning, she hurried towards the console.

'C'mon, if I can get back to UNIT,' she thought aloud. 'I can fix this; get help and-'

'Heya!'

Yaz jumped, turning.

A hologram of the Doctor had appeared. Same hair, same clothing, same smile.

Yaz let out a small squeal of alarm.

'If you're seeing this, I'm probably dead,' said the hologram. Yaz stared at them, her mouth falling open. 'Oh… bit overdramatic, maybe? Anyway, I have been working on this literally forever! It's a fully interactive holo-me! It's based on my behaviours across thousands of years, and it will only activate in the gravest of circumstances. Like if I'm not around. Anyway, I will have remembered to insert a nano-trigger under your skin at some point. I should do that; I need to remember to do that.'

'You could have told us you'd done that!'

'Whoa!' the hologram said, wobbling. 'The emotional receptors on this AI are a little oversensitive. Apparently, you're… upset with me? I should… apologise for something?'

She's lucky she's cute, Yaz thought, sighing. Sometimes she really does take the mick.

'You locked me in the TARDIS,' she said, irritably. 'Trying to keep me safe because you'd know I'd follow you into anything!'

'Oh…' the hologram replied. 'Er… sorry.'

Yaz turned and frantically tried to operate the TARDIS controls, but they didn't seem to budge.

'Did you set them on an automatic course?'

'Probably,' the hologram said, from behind her. 'I've got the TARDIS controls linked up to my sonic, so I can remote operate it if need be.'

'Where's it sending me back to?'

Yaz stared into the monitor.

Sheffield. Present day.

Of bloody course it was.

She'd sent Yaz home. That stupid, self-sacrificing time lord had sent Yaz home. Away from danger.

Away from the Doctor.

Yaz sank slowly down the floor, turning slightly as she did so, her back leaning against the central console. She put her face in her hands, and closed her eyes.

'Please…' Yaz whispered. 'I need to help her. She's surrounded by Daleks and Cybermen, and the Master as well. I… she needs me.'

The TARDIS made a soft bleep, and Yaz's eyes flicked open.

Then, two new holograms appeared in the centre of the console room, as if being projected from the TARDIS data banks. Two figures; one was a tall man in a leather jacket, the other a shorter, blonde woman in baggy flared jeans.

'This is emergency protocol one…' spoke the man, in a Mancunian accent. 'Now, Rose; listen…'

'Gosh, that's me,' said the hologram of Yaz's Doctor. 'Haven't seen that face for a while.'

'That's Rose, isn't it,' Yaz said, putting down her hands and looking up at the woman. 'The friend of yours that Captain Jack mentioned.'

The hologram nodded.

Yaz had long suspected that she hadn't been… the first time the Doctor had fallen for a companion. She knew that there had been others before. She wasn't sure how she felt about that. But… well, this wasn't her Doctor. Technically, it was the same person, yes, but… Yaz had fallen for her Doctor. Not any of the others.

'Emergency programme one means I'm facing an enemy that should never get their hands on this machine,' continued the man. 'So this is what you should do…'

'But… what am I supposed to do?' Yaz said, looking between the two figures.

'And if you want to remember me, you can do one thing… that's all, one thing.' The figure turned to look at where the projection of Rose was standing. 'Have a good life. Do that for me, Rose; have a fantastic life.'

Yaz stared up at them both. There was something strangely calm about the face of that Doctor. In the face of presumably overwhelming odds, they were still calm, asking nothing other than that Rose be safe and happy.

Which would have been fine, if it didn't remind Yaz of what her own Doctor had done.

'You can't do this me,' Rose exclaimed, her voice cracking. 'Take me back! Take me back!'

The holograms of the other Doctor and Rose disappeared. Yaz sat still for a moment, processing this. The Doctor clearly had always been this self-sacrificing, but that didn't make her feel any better. She could completely understand Rose's anger and pain.

'But what happened to her?' Yaz said, turning to look at the hologram of her Doctor, which had sat down next to her. 'Jack mentioned she was living on a parallel world, but that doesn't explain how she saved the Doctor.'

The hologram of Yaz's Doctor flickered.

'Yaz… I can't tell you. But… please don't do that. Don't do what Rose did.'

'Save you?' Yaz said. 'I have to! You'd do the same for me!'

'That's why you shouldn't,' replied the hologram, looking terrified. 'Please… just forget me.'

The hologram turned off.

'Not bloody likely!' Yaz exclaimed, jumping to her feet. 'You're my Doctor and I will save you!'

The TARDIS console gave a groan from behind her, and a panel opened. Yaz turned, getting onto her knees. She looked into the streaming light that emanated out of the opening. Her heart seemed to beat in her ears, and her mouth fell open gently. The TARDIS spoke to her.

She saw the past. She saw the present. And she saw the future. She saw it all.

'Forced regeneration,' the Master said, staring at the Doctor. 'The time lords even did it to you once, didn't they? That's how you ended up as that dandy with the white curly hair-'

The Doctor's brain was scarcely able to process this, before she automatically startled. The noise… that noise!

Was that-

'TARDIS materialising!' shrieked the Dalek commander.

The Master swung round.

'Bah! You cannot escape, Doctor!'

The doors of the TARDIS slammed open, and a bright light exploded out of it. It was so dazzling that it was almost blinding.

Yaz was stood in the doorway, her eyes glowing.

The Doctor felt as if both her hearts had stopped. No… no, not again…

Yaz floated off the ground and soared across the room, coming to a stop in front of the Doctor. With a crash, the constraints around the Doctor fell to the floor. She staggered forward.

'Yaz!' she cried. 'What have you done?'

'The TARDIS called to me…' Yaz said, her voice strangely ghostlike, as if speaking from a point faraway. 'And we looked into each other…'

'Yaz, you looked into the time vortex!' The Doctor collapsed to the floor by Yaz's feet. She stared up at her friend. 'No-one's supposed to see that!'

'What is this?'

The Master was staring at Yaz, his face contorted with a horrified expression, as if every fabric of his being was telling him to run. Run far away as fast as he possibly could.

He pointed the gun at Yaz, and fired.

The Doctor screamed, but the lazar halted in mid-air as Yaz turned to look at it. The lazar was forced back, back into the gun, which exploded in the Master's hand.

'Bad Wolf…' Yaz said. 'Yes, that's what she called herself…'

'Yaz, please!' The Doctor cried, 'You've got to stop this; you need to stop this now! You've got the entire time vortex running through your brain! You're gonna-'

'I want you safe,' Yaz said, and for a moment the light faded from her eyes, and her voice became like Yaz again. Tears began to trail down her cheeks. 'My Doctor. Safe.'

'You cannot defeat me!' spat the Master, clutching his injured hand. 'I am a Time Lord; I am far beyond your-'

'You are tiny!' Yaz said, her eyes glowing once again as she turned to look at him. 'I can see the whole of time and space; every atom of your existence, and I will not have you hurt my Doctor anymore!'

With this, she raised a hand, and the Master was thrown back several feet. He hit the wall, which collapsed around him, covering him with rubble.

The nearest Dalek sent forth a beam of deadly energy, but Yaz raised her other hand, sending the energy back into the Dalek, which exploded. The very particles of every Dalek and Cyberman in the room began to dissolve, leaving nothing behind. Soon enough, Yaz and the Doctor were alone in the room.

'Yaz, you've done it…' the Doctor said. 'Now stop.'

Yaz did not respond.

'Yaz… please! Just let go!'

Yaz did not respond.

'Yaz, you can't control the universe like this! No-one can!'

'I… I know,' Yaz said, and the light faded again from her eyes. 'But… it hurts, Doctor. It's gonna kill me. How can I…'

'You're Yasmin Khan,' the Doctor said. 'You can do anything. It wouldn't be the universe without Yasmin Khan.'

'Please… help me.'

The Doctor stood up and wrapped her arms around Yaz. Yaz's arms reach around her, and the two of them embraced tightly.

'Always will.'

Slowly, the Doctor took on part of the vortex energy, feeling it coursing through her like a conduit. Yaz slowly settled against her, as if a great weight was being lifted off the young Yorkshirewoman's shoulders.

The power, showing as more of the light, slowly drifted away, flying back through the doors of the TARDIS. Eventually, when all of it had returned, the light faded and the doors slowly swung closed.

Yaz and the Doctor stumbled.

'You did it,' the Doctor said. 'Yaz, you did it; you saved me.'

Yaz smiled, tears still running down her face.

'Couldn't have a universe without the Doctor, could we?'

Without thinking, the Doctor leaned forward, just as Yaz did the same. Their lips met.

The Doctor wished the circumstances could have been better. The two of them were both exhausted and drained, scarcely able to stand up. But she found it difficult to care. All she knew was that she should have kissed Yaz a long time ago. If she'd been braver, she would have been honest with Yaz from the start. But she knew as well as anyone that it was no good to think about what could have been. All you could do was make the best of the time you had. And that's what she was doing.

Slowly, they broke apart, and the Doctor was dimly aware of falling gently against Yaz. The young woman lifted the Doctor off the ground and into her arms, carrying her softly towards the waiting doors of the TARDIS.

The Doctor slowly drifted out of consciousness, her sole thought being that Yaz was safe.

The Doctor startled, and sat up.

She was lying on the floor of the console room.

'Ah, you're up.'

Yaz was stood next to the console, a wide smile on her face.

'How long was I…'

'A while,' Yaz replied, taking a few steps towards her and helping the Doctor to her feet. 'Long enough for me to stop and pick up Tegan, Ace, Graham and Kate; they all send their regards by the way.'

'You dropped them off home?'

'Yeah. Well… Croydon, so near enough.'

The Doctor chuckled, before staring into Yaz's eyes.

'You… you saved me.'

Yaz shrugged.

'You would have done the same for me-'

The Doctor stumbled. Yaz grabbed hold of her.

Clang!

'Why's the cloister bell sounding?' the Doctor gasped, looking around in confusion.

'Doctor…' Yaz said, slowly. Her voice was strangely hollow. 'Look at your hand…'

Slowly, as if moving through treacle, the Doctor looked down at her hand. Time seemed to slow down as she processed the sight that met her eyes. The sight that she had been dreading for longer than she cared to admit. Ever since the she had met personification of time. Perhaps longer, when she had started noticing the way Yaz looked at her. The way that Yaz smiled at her. Because she knew it would cause pain, and she hated that she knew she would be responsible for the pain Yaz would feel.

Regeneration energy was leaking out of the Doctor's hand.

'What?' she gasped, horrified. 'No… that's not right! I need more time! I want more-'

But she turned to look at Yaz, and a dagger seemed to go through each of the Doctor's two hearts.

Yaz was crying.

Tears were streaking down her face, and her mouth was trembling with suppressed sobs.

In that instance, the Doctor forgot about the regeneration, she forget about everything else, everything except Yaz.

'Yaz…' she said, softly. She cupped Yaz's face in her other hand. 'It's okay, Yaz… it's okay…'

Yaz's breath was ragged, as the sobs threatened to erupt. It broke the Doctors hearts to see her like this. Yaz was indomitable. Again, the Doctor cursed her own bad luck. Yaz deserved better than this. She deserved every happiness. But all the Doctor could give her was a small comfort, knowing full well that the ending was coming soon. That their ending was coming soon.

'One last trip…' the Doctor said, softly. 'Where would you go? What flavour of ice cream?'

Yaz gave out a small, teary laugh. Their eyes met for a moment, and then tears began to slide down the Doctors cheeks too.

'I did this…' Yaz mumbled, agonised. 'I made you take on the power of the time vortex-'

'Ssshhhh,' the Doctor said, pressing her forehead against Yaz's. 'It was an honour, Yaz. Truly.'

'You're dying…' Yaz sobbed. 'And it's my fault…'

'No; Yaz, none of this is your fault. I wouldn't let you die. Don't you remember? Can't have a universe with no Yaz. I can't have a universe with no Yaz. I promised I'd keep you safe, and that's what I'm doing.'

Yaz let out a miserable chuckle, as if responding to a joke the Doctor didn't think they'd made.

'So… how about that last ice-cream?'

Yaz sniffed, before smiling.

'Let me guess; no money?'

'Empty pockets.'

Yaz chuckled, before putting her arm around the Doctor's waist.

'What kind of date are you? C'mon, big head; ice cream is on me.'

And, holding each other, the two of them headed out of the TARDIS doors.