Terry rushed into the box after the Doctor, turning to glance through the door before slamming it firmly shut. He kept his eyes fixed on the door while turning his head slightly to speak to the man he assumed to be just over his shoulder.
"I think we lost them," said Terry. "Now I assume you have something better in mind than hiding in a cupboard because..."
He tailed off. Something wasn't right. The Doctor should be right there, the box was only a few feet across, but there was no sense of a body, no warmth, no pressure, and the air was moving..
He turned around.
The Doctor had already bounded up to the console and was gleefully playing with the controls. As the groan of ancient engines filled the filled the air - the ground, the door, Terry's bones - the Doctor shot him a glance.
"'Salright," he reassured his young friend, turning back to the controls, "We're safe now, we're in flight. You can relax. You can say it."
Terry took a breath.
He lowered an eyebrow and raised a finger.
The Doctor grinned again. "Ye-es?"
"... It's ... a teleport?"
"No, no it's not a teleport," said the Doctor, still grinning.
"It's a gateway, then. A portal interlock to your ... secret lair or ..." Had the Doctor said they were in flight? "Ship?"
"Yes, it's a ship, no it's not a portal." The Doctor's expression was more quizzical now, and he leaned on the console. He'd been expecting a simple exclamation, not an interrogation.
"Okay, the ship exists in another plane - like a hyperspace - and extrudes a physical exit into real space?"
"You really do watch far too much science fiction. Not even close."
"Right. So it's a pocket dimension - a tesseract? At right angles to reality?"
"No, no, no. Getting colder. Sub-zero. Kelvin. And that's not even possible."
"Alright, I give up. What is it?"
"You sure? You'll kick yourself."
"No, yeah. Tell me."
"Alright. What it is, is..." The Doctor beckoned Terry closer, looked theatrically around to make sure none of the other imaginary passengers were listening and lowered his voice. "The way it works is..."
"Yes?" demanded Terry.
"It's bigger ... on the inside."