Despite returning in triumph bearing the anaesthetised Master, the Doctor managed to beg off any further celebratory ceremonies. He made his good-byes. Councillor Romana saw them off.

"Are you coming with us?" asked Rose.

"Oh, no," laughed Romana. "I travelled with the Doctor once...but that was a very long time ago."

The Doctor gazed at her fondly for a moment; then stopped and looked about, clearly confused. "Where's my TARDIS? I left it right here."

"Oh, that's a surprise," said Romana, smirking. "A token of our gratitude. We fixed the chameleon circuit. It is now completely invisible."

"But I didn't want the circuit fixed--I like the police box! You know that very well, Romana!" cried the Doctor agitatedly. "How am I supposed to find it?"

"You'll manage, I'm sure. Good-bye, Doctor. Good-bye Rose." Romana waved and left.

Muttering irritably to himself about upstart, impudent, teasing Time Ladies, the Doctor began feeling about for his TARDIS. "OW!" he cried, stubbing his toe. Then he disappeared.

"Doctor?" called Rose. "Where are you?"

A vertical sliver of TARDIS interior appeared in mid-air, then widened. "Right here," said the Doctor, hopping on one foot in the doorway. "Watch your step."

"I have a question," said Rose, as she made her way inside.

"What is it?" answered the Doctor, as he began checking the TARDIS controls.

"I still don't understand where all those parasites that caused the epidemic came from. It was something to do with the shield, I know, but...you said it wasn't a bioweapon."

"One of the perils of time travel is that sometimes you pick up hitchhikers." He winked at her. "Only in this case, the parasites were hitching a ride on Rassilon's shield. Two million years ago, I suspect they were everywhere, but Gallifreyans had a natural immunity to them. But since then, the parasites died out and consequently our immunity waned. Which reminds me--" He snapped his fingers. "If I ever take you back to pre-20th century Earth, we've got to make sure you've had all your shots."

"Okay..." said Rose, not at all enthusiastic about the prospect.

"Now, where to, where to?" said the Doctor, stroking his chin. "Enough of boring, crusty old Gallifrey, how about--"

"I have another question," Rose interrupted. "What is 'parallel pressure'? You talked about it when you were delirious, and the Master said it would make it easier for him to destroy the Time Lords."

The Doctor's expression turned grave. "It is a property of parallel universes. Every time you make a decision; every time an event could go one way or another, it draws in the Void and creates a new parallel universe, one for every possible outcome. Thing is--most decisions turn out pretty similarly in the end. So the divergent parallel universes eventually collapse together again, driving out the Void. Are you with me so far?"

"Er...yes," said Rose. "Sort of."

"Very large events, however, such as the death of the Time Lords in one universe, create a sort of 'pressure' on adjacent parallel universes, pressure for a similar event to occur in the adjacent universe, so that they can collapse together again in the future. I was concerned that the contagion was a manifestation of this pressure, perhaps leading to the weakening and death of the Time Lords in this universe."

Rose puzzled over this for a moment. "But...if the Time Lords are alive in the other universes, and mine is the only one where they're gone...shouldn't the pressure be in the opposite direction? Shouldn't there be pressure for the Time Lords to return?"

"Very good, very clever," said the Doctor, smiling. "Yes, and that is a happy thought."

Rose nodded, then said, "I have one more question."

"I had a feeling you might."

"It's something the Master said. Can you really remember the future?"

The Doctor laughed. "What a preposterous idea. You are a very silly girl, Rose Tyler."

Rose scowled. "Well, I just thought...hang on! I never told you my last name is Tyler."

"Of course you did."

"When?"

"Just now."

"Doctor!"

Many thanks to everyone who has taken the time to review. I write principally for my own amusement, but it's great to be able to entertain others as well.

Stay tuned for the next story in this series: "The Restaurant at the End of the Multiverse"