River was overjoyed when she learned Amy had regained her memory—mostly because it was Ian's lunchtime and he was threatening to kill her. As soon as he was returned to his mother's arms he settled down, and Amy hugged him tightly, promising to never forget him again.
"Right, so, we're going to return to the scene of the crime and see if we can discern Bai's whereabouts," the Doctor announced.
"But you said poltergeists are difficult to recognize sometimes. What if he doesn't want to be found?" Amy asked.
"Poltergeist? That would make sense," River remarked, recalling the boy's inhuman behavior.
"Oh, he wants to be found, alright. His kind have a very strong sense of curiosity, and right now he's wanting to know what I am and how I knew who he was. Shouldn't be a bit of trouble."
A little while later, Amy wasn't so sure. She was standing next to the Doctor, a drowsy Ian in his carrier again, surveying the debris of the caved-in gymnasium ceiling. Tables were turned on their sides, their displays scattered over the floor. An unseen toad—perhaps part of an exhibit—croaked nearby, his raspy voice echoing eerily in the silence that permeated the air. The baking soda and vinegar volcano would have looked innocent if it wasn't still smoking slightly. "Well we tried. Let's go," Amy said lightly, making for the door.
The Doctor grabbed her by the arm before she had taken more than two steps. "Not yet. He's here."
There was a distortion in the shadows, and Bai Wu suddenly appeared, hovering a few feet off the ground. He landed in front of the Doctor, his shoes not making one whisper of noise as they touched down on the over-glossed floor.
"So you came back," said the boy in the voice that didn't seem to fit, his lips curling.
"Indeed we have. How've you been, Bai? Blown up any other educational facilities while we've been gone?" The Doctor said as if making polite conversation.
"Haven't had the time. I was a little preoccupied with learning who the man who knew my secret was," Bai answered.
"Yeah, and almost killed my wife while you were at it. Needless to say I am not very pleased with you right now."
"My choices were limited. She was my only way in."
"I don't care." The Doctor squared up to the boy, staring him down. To an ignorant bystander it would have looked like a grown man challenging a helpless child, but in truth they were equals; one ageless being who had gotten off on a very wrong foot with another. "She's off limits, understood?"
Bai narrowed his eyes, and for a moment he looked as if he might defy him. But then he stepped back, his hands raised. "My apologies, sir. I was not aware of her intrinsic value."
The Doctor dipped his head, curtly acknowledging the apology.
"Now, sir, you seem to know what I am, but I do not yet know what you are. You are not from this world," Bai said.
"No, I am not. Originally from Gallifrey, go by the Doctor."
"Ah, a Time Lord. That would explain it."
"And your name isn't really Bai Wu, is it? Just a handle you adopted while you're in this form."
"Correct. My real name is hidden, as yours is. Names have power."
"Yes, as demonstrated by the Carrionites. What's still a mystery to me, however, is why you chose this particular form, and the reason for that little volcano fiasco earlier today."
"Straight to the point, Doctor, humans fascinate me. The way they live and survive, despite the circumstances. Thirty years ago I came to this school as a student in order to observe them." Bai's expression turned sour.
"And they ridiculed you." The Doctor knew enough about the school system on Earth to understand that someone as out of the ordinary as a poltergeist wouldn't be accepted, disguised or not. Schoolchildren were schoolchildren.
"Ridiculed? If only. They tormented me, ripped me apart. So I sought my revenge. I came back."
"To blow up a gym? Impressive," River remarked sarcastically.
Bai grinned. "That was only the beginning. They shall suffer for what they did to me, make no mistake."
"But these children aren't even the ones you went to school with. Why punish them?" Amy asked.
"They're all the same; children. One as nasty and hateful as the next, always bullying. Besides, these children happen to be the offspring of my tormenters, and they're no different from their parents. Honestly I'm doing the world a favor."
"Oh, a favor. Is that what you call it?" Said the Doctor. "Tell me, Bai, is destroying a secondary school really worth it? Your kind live for centuries; why bother seeking revenge for scars time will soon heal?"
"You should know better than that, Time Lord. Poltergeists don't take kindly to being mocked. When someone does us wrong, we get retribution. And it won't be just on this school, no. I've had thirty years to stew, to prepare a perfect plan of revenge, and I will not stop now, not until every child on this stinking planet has suffered my wrath."
The Doctor glanced over his son, imagining him older, and fully human, being sent off to school with an excited smile on his face. There had to be millions of children in the world who resembled that image, all of them innocent and ignorant of the danger that awaited them. His eyes met Amy's, and he knew she was thinking the same thing.
"That truly was a brilliant speech, but there's something missing. There are countless schools on this planet—even more children—and only one of you. How's the suffering-of-wrath bit supposed to work out, eh?"
"Like I said, Doctor, I've had thirty years." Bai strolled over to his volcano, which seemed to be the only thing left untouched in the room. "Do you see this glorious device? Quite remarkable how such a small thing could have the power to cave in an entire gymnasium roof, wasn't it? And that was just on the lowest setting. At full capacity, it can create a nuclear-like effect. Now, I've planted quite a few of these beauties in schools throughout the countries of this world. No one pays much attention to a baking soda and vinegar volcano, do they? At least, not until it's too late."
"You wouldn't," River gasped.
"I already have, dear lady. All it will take is one touch of this button. Just one." Bai procured a small remote control-like object from his pocket, which was fitted with a single bright red button. "Tell me, Doctor, how would you like to see your precious little family turn to dust? That'd be quite a sight; I daresay it would torment you for the rest of your long life."
The Doctor heard Amy's breath catch in her throat, felt her pull back a step. His temper flared, but he forced himself to stay calm. "And what will you have gained from it, Bai? A small glimmer of satisfaction, before loneliness sets in again. Why not just give humanity a second go? I promise you there are people in this world, good, kind people, who are worth getting to know."
The coldness in Bai's eyes didn't waver even for a second. "They've had their chance. They don't deserve another one."
"Please, Bai. Don't make me have to stop you."
Bai guffawed. "Now that is rich. Think just because you're a Time Lord you can stop me? Have you forgotten what I am?"
"I warned you," the Doctor said. In a blink of an eye he had whipped out his sonic and clicked it. The detonator remote all but jumped out of Bai's hand, hitting the floor and shattering into a thousand pieces.
Bai shrieked in indignation and grabbed at his hair, pulling out a few clumps in the process. "You will pay for this, Time Lord! Beware, for I will return, and take from you what you treasure most!"
There was a crack, and the poltergeist disappeared, a cloud of rancid black smoke left swirling in his place.
"Is that it? Is he gone?" Amy asked, her nose wrinkling from the smell.
"For now. But he promised he would return, and poltergeists always keep their word. We'll just have to stay on our guard," the Doctor replied grimly.
Note- the Carrionites were in the episode The Shakespeare Code, with the Tenth Doctor and Martha.