"So, what is it" Dorothy asked as the Doctor piloted his ship, "Hypnosis? Body snatchers?"
"Nothing so straightforward," the Doctor replied, half-distracted by a dozen calculations he was doing in his head. He pulled a shiny object out of his pocket and held it, examining it, "you see, long ago on our world, there was a group of people who believed that we could be more than we were. That we should develop our capacity for mental influence, telepathy, telekinesis, without holding back in the name of decency. They wanted Time Lords to rise to become the most powerful beings in the universe. They were called the Infinity Syndicate, and many members secretly held positions of power in the government."
"And Cara got mixed up with this lot?" Dorothy frowned, "that doesn't sound like her type of party."
"They found her once while she was regenerating," the Doctor sighed, "that's how they would recruit members, because no Time Lord would join them by choice. But when we regenerate, we're vulnerable. While her mind was still developing, they went inside it and changed things. Subtle things. They left feelings and suggestions that would make their cause seem logical to her, even morally right."
"That's awful!" Dorothy gasped.
The Doctor turned the object in his hand, "when Gallifrey fell, so did the Infinity Syndicate, all except Cara, who was left with one instruction." The Doctor held up the object, a silver ring fashioned into a smooth figure-eight symbol, "Redemption, by any means necessary. And now," the Doctor stared past the ring at his TARDIS, "they've given her the power to raise all the Time Lords from the dead. To raise an entire army of vengeful beings with the power to destroy anyone just by looking at them, and to turn them loose on our enemies."
"Wow," Dorothy said, unable to think of more.
"Trouble is, they won't stop there," the Doctor shook his head, "when the time lock around Gallifrey breaks, it'll send shockwaves across time and space, drawing my people outward, to everywhere there's a civilization."
"Crazy mutant grudge-holding Time Lords everywhere," Dorothy blinked, "sounds like a problem."
"And we're the only ones who can stop her," the Doctor sadly looked at the ring again.
"Wait," Dorothy eyed the Doctor nervously, "we don't have to, you know…kill her, do we? I mean, she saved my life once…"
The Doctor quietly checked his controls, "I won't destroy her, but I may not be able to save her." But I've got to try.