We're nearing the end now, but not to worry, I have another plot bunny just ready to take over my life :)
"You can do something, can't you Doctor?" He was standing at the control panel of the TARDIS, doing the Time Lord equivalent of Googling the Picarion.
"I think so. I don't know. I've never heard of these guys before. I don't know what they need."
"You said they were like Magpies, right?"
"Yeah, grabbing onto the shiniest brain and strapping themselves in."
Rose bit her lip.
"I need to talk to it again." The Doctor suddenly decided. He strode over to Daisy, who was eagerly reading some literature from the 30th century from the TARDIS library. "Daisy?" he interrupted.
She looked up at him, all traces of seriousness gone as she clutched the book to her chest.
"I need to talk to the Picarion. I need you to let it take control of you again."
Daisy's expression changed to one of worry.
"How?" she asked. "I don't know how."
"Is your head aching?" the Doctor asked.
"Only a little."
"Okay. Concentrate on the pain. Really focus on it. Close your eyes and let the pain spread."
Daisy hesitated, but did so. After just a few moments she slumped forward, blacked out from the pressure. The Doctor caught her as she did so and propped her up against the chair. Reaching into his pocket he pulled out his sonic screwdriver and aimed it at her forehead.
"What are you?" he muttered to himself.
At the sound of the Doctor's voice, Daisy sat up straight, the Picarion visible in her eyes.
"I thought you'd come back." the voice smiled through Daisy.
"What are you? And how come I've never heard of you. I've heard of everything."
"So modest, Doctor." the Picarion mocked. "I suspect the great and powerful Time Lord has never heard of us simply because we are too lowly for him to notice. Whilst you travel around in your box, your... TARDIS, we have infiltrated this planet, the planet of your favourite little ape."
"Excuse me..." Rose began to interrupt before she was silenced by one look from the Picarion.
"Ape. Silly little ape, thinking you know so much because you have travelled the universe, with your precious lover. You know nothing."
"Leave her alone." the Doctor warned in his most menacingly quiet voice.
"Oh isn't this sweet. Trying to protect her when you know in your heart you cannot."
"Oh I can protect her. Especially against you."
"You barely know us."
"You keep saying that. Us. And we." Rose began, trying to avoid looking directly at Daisy's possessed body. "But you said you're the last. That there aren't any of you left. You're on your own."
"I am the last of my kind. My race was destroyed many years ago. But I have my Host, this clever, clever girl."
"Daisy?" The Doctor leaned closer to her, peering into the girls eyes. "Daisy I know you're in there, and I know you can hear me. I need you to do something for me. I need you to look into your own head. You can do it, so don't even try to say you can't. Use your brain to look at this thing. What is it? Find out whatever you can and then pull it back. Pull it back and tell me."
With some more ease than previously, Daisy subdued the Picarion and found herself back in control of your mind.
"Well, what did you see?" the Doctor questioned eagerly, having no doubt that the immense brain power that the Picarion had helped Daisy achieve would be its downfall. Combined with his own considerable intellect, they couldn't fail. And then he could tell Rose. That was his prize, and his fear.
"It's gaseous. It floats in the space between the hemispheres, seeping into both sides. It has carbon in it's make up. Carbon and hydrogen and something else. Something that I couldn't see. And there's a taste. It's sweet."
"Like sugar sandwiches and cake?" the Doctor suddenly questioned
"Yes... Now I think about it, precisely like that."
"That's what it's been using!" the Doctor grinned manically at both women. "Sugar!"
When both Rose and Daisy simply stared at him he launched into his characteristically exuberant explanation.
"The Picarion, it's using enormous amounts of energy to keep itself it its gaseous state and to stay in your head. It's been using sugar. It's brilliant. The sugars both keep it in a non-natural state, and give you the energy of... well... me, in order to do the absurd amount of studying you need to do for it to become powerful enough to take full control of you and, eventually, through you, the entire earth."
"So if she just doesn't eat sugar, she'll be okay."
"Well, yes technically. But there's an even quicker way. I think I might know how we can do this, with little damage to your brain Daisy."
"And then I can go back to my research."
"And you can go back to being wonderfully, incredibly, overly smart. But first, and most importantly, I'm going to need a lot of cake."