While the little people were accomplishing their tasks in the TARDIS (of course, the Doctor hadn't forgotten to warn her to keep an eye on five teeny tiny Masters who could create any kind of no good things), the Doctor was exploring the outside room. No doors expect the mentioned darkened rectangle, no windows, no holes of any kind. He even moved some of the full-size and tiny furniture around, but that didn't help much except for discovering some long-forgotten clothes under the small beds and a tiny bathroom in the corner - the Doctor could hardly push a hand through the door to it.
After walking for a while along the doors the Doctor finally noticed something. The dust wasn't lying smoothly on all the surfaces - it was collected next to one wall, as if blown by non-existent wind to there. Hmm, that's interesting. The Doctor pressed his hand to that wall, and in some seconds it became covered in a thin layer of dust. Something was pushing it there.
The Doctor decided to check one more guess of his. Actually he would have gladly perform this experiment by himself, but here there was nothing like a chair of a needed size for him, and searching for it in the TARDIS could have taken too long.
"Anybody here?" he called into the blue doors. "Anybody free? I need one of you for a minute!"
Guess who was the first to come? Captain Jack Harkness, who else! His big big blue eyes made him look like an innocent little creature, but the Doctor knew the potential of mischief in him.
"Come here". The Doctor brought one of the small beds approximately to the middle of the room. "Climb onto the top".
Jack shrugged, but obeyed.
"Now jump down here". The Doctor cupped his hands next to the bed. "Don't worry, you will be safe. Jump".
He didn't have to ask twice - in a mere second Jack landed onto his palms. But what was that? All his cheekiness and pride couldn't hide the fact that the jump affected him, and greatly, so that the chibi stayed in a sitting position in the Doctor's hands, rubbing his head.
"Did that hurt?" the Doctor asked. "I'm sorry". Jack meeped, showed a pinch with his fingers and, slightly wobbling, returned to the TARDIS.
Okay, the Doctor thought. Those gravity effects were active for just one kind of place.
"I think we're on a giant spaceship", he said aloud. "Now we know where we are. It's only left to discover WHEN and WHY we are here. In the name of Bowtie Nebula, I'd rather kill myself if we're going to be in the TV show again..."
His thoughts were interrupted by a shriek of horror from the TARDIS doors.