A/N : Thought I might change it up a bit, so this chapter is going to follow the Doctor instead of Missy. -L x

Guilt weighed the old man down as he walked back to his TARDIS, with Clara following him attentively. They had stayed in silence during the boat ride, which had seemed like a good idea at first, but now it had been four hours, and the Doctor was now doubting his motives.
"Okay, it's official. Silence is even worse in a Scottish accent", Clara exclaimed, trying to keep up with him. "Are you going to tell me what you're brooding about?"
"It won't stop, the repair kit I put inside Ashildr, not ever. It'll just keep fixing her", he looked back at her, her face one of hopefulness, but also concern.
"Well, good".
"I'm not sure, but it's entirely possible she has lost the ability to die".
"The ability?'
"Oh, dying is an ability, believe me. Barring accidents, she may now be functionally immortal", he started to unlock the TARDIS, but was surprised to see it was already open.
"If the repair kit never stops working, then why did you give her two?"
"Immortality isn't living forever. That's not what it feels like. Immortality is everybody else dying. She might meet someone she can't bear to lose".The Doctor bent down to the ground, noticing the white handkerchief floating around on the ground. He picked it up, looked at it for a moment and put it in his pocket.
"That happens, I believe".
Clara followed him into the TARDIS swiftly, as he looked around for any signs of damage she could have done. Cautiously, he logged into the TARDIS database and saw that some parts of it had been hacked via sonic. "What she done, old girl?" he thought. Most of his files had been either hidden or deleted, and he saw that a download bar had completed. "Oh, she better not upload that dancing video to the Galactic Hub", he murmured to himself, seeing Clara was watching him attentively, like he was some sort of science project.  "I was angry. I was emotional. Just possibly, I have made a terrible mistake. Maybe even a tidal wave". The TARDIS whirred as it dematerializes.
"Time will tell, it always does". Huffing, he sat down on a metal step with his head in his hands.
"Whatever you did for Ashildr, I think she deserved it".
"Yes. Yes, she did. But Ashildr isn't just human any more. There's a little piece of alien inside her, so in a way, she's". He paused for minute, hearing Davros and Missy's voices blaming him inside his head. "In a way, she's a hybrid". Suddenly one corner of the console room glowed and whirled, the Doctor squinted past the blinding light to see two silhouettes. The blue then dropped to reveal Missy, with wild curls rolling down her shoulders and plum skirt ripped up the side. She was wearing dark stockings underneath and her face was twisted into a scowl. In her arms lay the still unconscious Donna, still in work clothes, but with a sonic held to her temple. "Yes, she is Doctor. I warned you", she shouted.
"Koschei, put the sonic down. Don't bring Donna into this", he said, sounding strange to Missy to hear her Gallifreyan name roll of his tongue.
"Doctor what's happening? Who's she?" Clara asked, worried.
"That's Donna Noble, one of my previous companions. She absorbed regeneration energy and became a biological meta-crisis. I had to delete her memories of our time together to save her. But what that doesn't explain is why you're holding her hostage with a sonic to her head? What do you want?"
"She loved you, you know? I looked inside her head, she still remembers you. But you're known to her as the Unknown. An alien she fell in love with, but forgot about".
"What are you talking about?"
"You know what I could do, don't you?"
"Yes, you could reignite her memories, but it would kill her. You wouldn't do that".
"Dr. Chang, Seb, Osgood, one tenth of the population of Earth, 33,200 Daleks say otherwise".
"What do you want then?"
"Who says I want anything dear?"
"Well you're holding my previous companion at gunpoint. It doesn't take a lot to see that you're threatening me. So, what do you want?"
"My daughter. Well, my daughter, your human guinea pig apparently".
"...what?"
"Ashildr. She's my daughter". She cocked her head sidewards, digging her sonic further into Donna's head. "Don't you remember Doctor, how you made her forget me, her own mother? How you chucked her into the nearest timezone for some mortal to pick her up? And then just to top it off turn her into a Dalek/Time Lord hybrid when my back's turned?"
"...the Time War... I had to... Your mind would have passed onto her".
"SHE DOESN'T KNOW WHO I AM!" She screamed at him, her eyes filled with fury. "But your companion is surely going to know who you are". She flicked her sonic at Donna, and with one small buzz sent her breathless to the floor holding her head.
"No, Missy, please no".
"That was just to wake her up. One more and I'll break her quicker than you've broken my family". Donna slowly pulled herself onto her knees, looking up at the people she had no idea was.
"Where am I?" she asked Clara, who seemed to be the happiest in the room.
"Don't worry, you're just being held hostage for a minute", she replied, trying to crack a half-smile.
"Ashildr. Now", Missy called, ignoring them.
"No, she's in a place where people love her, respect her, won't turn her into a weapon".
"Doctor, I am this close to shooting you".
"Go ahead, might take you a while".
"Seriously, you two are arguing like an old married couple. What the Hell are you two on about?" Donna interrupted, wobbling as she started to stand, held up by Clara.
"Custody over our, I mean my daughter".
"Hang on, 'our'"
"No, no I meant my. Just a slip off the tongue".
"That's why I knew her. My own daughter and I forgot".
"No, it's my daughter. You know what, just come here and you'll forgot all about it". She rushed past Donna, almost making her fall back to the floor again and crashed into the Doctor's lips. He almost welcomed it, until he realised what it really was.
"Stop it! Stop wiping my memory. Now tell me straight or I'll telepathically pull it out of you-"
"Saucy..."
"As I was saying, is she my daughter?" Missy wandered away from The Doctor, unable to look at his wondering eyes.
"Yes, she is..."
"Do you love me?"
"More than anything in the world Doctor. But, like you said, I can't be part of a family, I'll just turn her into a weapon".
"So what now?" The Doctor asked, putting his hand on her shoulder.
"Well, I guess I'll take Donna back, remove any traces of you and just disappear again..."
"I want to know", Donna whispered, now standing up on her own, but without any sign of Clara. "I want to know who you are, who I am? I've never known. It's always been lost somewhere".
"I can't Donna, you'll die".
"If I led another life. A life I never knew, with a man I loved more than anything and adventures I could never dream up, I'm already dead".
"Donna!" called Clara from a set of steps. In her hand was the Doctor's sonic screwdriver. "Do you want this?"
"More than anything".
"Clara, do you want this?" asked The Doctor, still wondering where she got his sonic from.
"What, to save a life?"
"No, to destroy one".
"You're a coward, you know that?"
"Runs in my veins, my impossible girl".
"She said herself she's dead already. How would you feel if you couldn't remember your life?"
"I wish I could. My life's full of mistakes, and sometimes..." He lowered his head and looked at Missy. "I wish I was someone else". Missy raised her eyebrows at this snarky response.
"Really?" She asked, circling him like a leopard with its prey. "You'd prefer to be a psychopath, forced by her own kind to kill and kill and kill, without remorse, forgotten by her family and shunted by her friends. Stop running away from your mistakes, pitying yourself, and try and fix them for a change".
"Fine! Do it, bring her memories back. But if she does, it's on your conscience Clara Oswald, yours".
"I'm sure I have more on it Doctor", she replied, and buzzed the sonic at Donna. She held her head and breathed heavily as she fell backwards into the chair again.