Yaz had faced many unique threats and dangers in her time. Sontarans. Weeping Angels. The Cybermen and of course the Daleks.
Until she found herself attending a 'black-tie' party that was later besieged by a mutated scientist who had attempted to meddle with the natural human life cycle. Oddly enough, Yaz felt slightly more at ease when the latter transpired. A fight for her life was at least something she was familiar with, as opposed to remembering everything about being polite and proper that was required at human social gatherings.
The mutated Professor Lazarus had pursued her and the Doctor through the building. Their attempts to destroy him, the Doctor igniting the flammable gas in one of the labs and Yaz throwing a compound of various corrosive acids, had done little more than irritate him. It was only when the Doctor managed to reverse the polarity of the Professor's sonic micro-field manipulator that they managed to subdue him. The resulting blast of energy had incapacitated Lazarus and apparently stabilised his fluctuating DNA, restoring him to human form.
In the aftermath, both Yaz and the Doctor removed their bow ties. The Doctor began to address Yaz's mother, even Yaz could have predicted the slap that she gave the Doctor simply by the look on her face.
"Keep away from my daughter!" she snarled.
"All of the mothers, every time," complained the Doctor.
"Mum, what are you doing?!" demanded Yaz.
"She's dangerous," she said. "I've been told things."
Yaz was perplexed. "What are you talking about?"
Mrs Khan gripped her shoulders. "Look around you. Nothing but death and destruction!"
"This isn't his fault. He saved us, all of us! He's a good man, she saved us from the Daleks" insisted Yaz.
However before anyone could say anything else, they all heard a loud crash. The Doctor looked at Yaz grimly before running off. Yaz followed without hesitation.
Their suspicions were confirmed on arrival. The ambulance transporting Lazarus had crashed, the attendants and Lazarus all missing. Using her screwdriver, the Doctor detected the distinctive signature in the nearby cathedral. Together, they went in to find him behind the altar. He was draped in a red blanket and contorting violently, accompanied by loud cracks of bone.
"Another man once said much the same as you." She approached the human slowly. "A scientist who altered his race in the name of survival and acting for the greater good. But it was merely to conceal his own ambitions, to place himself alongside gods, even though his creations would go on to bring devastation to all civilised worlds."
"Is that so?" said Lazarus.
"You should know. Not too long ago, they were raining death in London's skies." The Doctor knelt down, locking eyes with Lazarus. "I warn you, Lazarus. This is a dangerous road you are travelling down. There might still be a chance for you to turn back. I can help you."
She held out a hand to Lazarus. He looked at it as if it was diseased in some way.
"I will not allow," he growled.
"Lazarus, do not be a fool!" hissed Yaz
"You're the fools!" said Lazarus. He cried as he convulsed again. "If Only you realised it ."
The Doctor gently pulled Yaz back and stood before Lazarus.
"Fool you twice," he replied.
" Twice?" The Doctor answered her question with a knowing look and resumed circling Lazarus.
Lazarus laughed as he threw a key to the Doctor, the de guise finally coming off. The Doctor and Yaz stared in confusion.
" Come on Doctor catch up" he said.
" Oh for fuck sake " the Doctor said
The Master laughed before he teleported away.
" Thats all we need" Yaz whined.