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“What are you doing in here?”

The Doctor’s tone was as close to a reprimand that Donna had ever heard. She turned to stare at him, unsure if her queasy stomach originated from his glower or from the general atmosphere of unease pervading the room.

“I… I was heading for the kitchen and took a wrong turn.” She swallowed hard. “I’m in for it, aren’t I?”

The Doctor calmed himself down, stretching his fingers before he replied. “No. No, not at all, Donna. You’re fine. It’s just… it’s probably best if we go elsewhere.”

“Why?” she asked. “What is this place?” She glanced around at what looked like a warehouse of multiple aisles of shelves. Dark glass jars were packed together, like they were deposited in a hurry with no care for organisation, or even safety. “It looks like a chem lab stockroom.”

“No. It’s... Here.” The Doctor stepped to the nearest shelf and selected a jar. He popped its lid off and poured some of its contents into Donna’s hand.

“Sand?” The multicoloured grains sparkled as they fell through her fingers.

“No. Look closer.”

She spread what was left across her palm so that she could inspect a few individual pieces, and she realised that each bit of what looked like transparent jewels contained… something, too small to discern but constantly changing, churning within the tiny mote. “What is that, inside the sand?”

“This is a time repository.” The Doctor replaced the lid on the jar and put it back on the shelf before continuing. “I’ve told you before, my people had technology that could change time, either on a grand scale or maybe for just one person. They could change a planet, or a town, or a life, or pluck a person out of one timeline and put them into another. They could even make copies of someone and give each copy a different destiny.”

He twiddled nervously as he explained. “Well, each time you do that, you’ve got this leftover timeline, a discarded branch, a path never taken. So they would take that and store it somewhere. Somewhere like here.”

Donna frowned at him, then bent to look at the bits in her palm again. “Each of these is someone’s life? Or not-life, I guess?”

“Something like that, yes. A line of time. Not necessarily a lot. Just a bit that didn’t happen. No longer happened. Happened but was replaced.”

She looked around again, at the room so large that she couldn’t see the other end and at all the jars on the shelves. “There are billions of bits like this in here! You did all this?”

“No! Not a one!” the Doctor denied. “I don’t work like that.” He scrubbed at the back of his neck. “The TARDIS wasn’t always mine.”

“She’s seen a lot,” she murmured.

“That’s putting it mildly.”

Donna shivered. “Can we… I’d like to go now.”

“Yes. That’s probably for the best.”

Donna scurried out and the Doctor turned out the lights before following her and shutting the door.