"What's that?" the Doctor asked as he passed by Rory holding a book. Who reads a history book on the TARDIS when you can live it and see it in person?

"It's a book about parallel worlds. One chapter says that there are parallel worlds stacked on top of each other. Is that true?" Rory replied.

"Is 'what' true?" Amy asked as she entered the room. She took the book from Rory and began to flip through. "Parallel worlds…..they exist?"

"Yes, they do. I've several times to one particular world," the Doctor answered unhappily.

"Can we go to one?" Amy asked.

"No," the Doctor said strictly.

"Well, why not?" Rory asked. "Why can't we go?"

"Because you can only get there on accident, and besides….I already closed all the gaps to them. They're sealed closed," the Doctor said with almost no patience.

"You lost someone…didn't you," Amy said suspiciously.

"What are you talking about, Amy?" Rory asked.

"I'm saying that he probably lost someone in a parallel world. That's why he doesn't want to go," Amy said as she turned to face the Doctor. "Who was she?"

The Doctor didn't answer her. He just stood there looking at the TARDIS console waiting for this conversation end. Meanwhile, Rory kept searching through his book to learn more about parallel worlds. Why did they want to go so badly? What's wrong with the one they're in right now?

"Okay, fine. At least tell me her name. Doctor…please. Just tell me her name. That's all I ask," Amy pleaded. She just reminded him of her so much. The Doctor just couldn't say no to her.

"Her name was Rose; Rose Tyler. Now that's all you're getting out of me, so drop it," the Doctor demanded.

"You mean there were others?" Rory asked as he put down the book.

"Of course there were. I'm 907. Of course I've traveled with others. I've had lots of companions, and each and every one of them have an equal spot in my hearts. Rose was just…special, in a way," the Doctor said as his mind drifted to old memories of Rose.

"Did you love her?" Amy asked. "You did. I can sense it. Let's open up those gaps and pop by for a visit. What's so bad about that?"

"It's hard to have to leave or lose someone without a goodbye. It took me 3 months to finally tell her goodbye, but I didn't get to finish. It took her two years to get back to me, and when we said goodbye again, she didn't want to leave, so I gave her the human clone that had accidently been made of me. It was me when I met her; full of rage and anger...waiting for someone to hold him back. I told her that he needed her to make him better. Then I had to leave. It's hard to say goodbye once. If I have to keep saying it, when will the meaning sink in?"

"Is she happy?" Rory asked the Doctor as he sat down next to Amy on the TARDIS staircase.

"What do you mean?" the Doctor asked puzzled.

"Rose, is she happy? Is she happy where she is?"

"Absolutely," he replied.

"Then that's all that matters. It's all you want for us, and that's what she is in the parallel world; happy," Amy answered. And she was right. The Doctor only wanted happiness for his companions and the one named Rose Tyler.

"If I try hard enough, I could get us to the parallel world so you could meet her," the Doctor said after a long silence.

"Thanks….but no thanks," Rory said. "We're fine. Really, we are. Why put you through the trouble of another goodbye?"

That night when Rory and Amy had gone to bed, the Doctor snuck out of bed and made his way to an old bedroom that had been empty for several years now. It hadn't been occupied since she left. The Doctor had been in there several times when Martha traveled with him and a couple times when Donna explored the TARDIS, but not once had he visited the room while he traveled with Amy and Rory. The Doctor decided it was time for another visit. He opened the door and looked at the wall of pictures hanging up that had never been taken down. They all had the same person in each photograph. Someone who would stay in the Doctor's hearts for the rest of his long, long life: Rose Tyler.