Northern Germany, Earth, c. 150 000 000 BCE

"Welcome to the late Jurassic," Helen announced as they stepped into the clearing. "Do a lot of archaeologists in the fifty-first century get a chance to do this sort of thing?"

"No." River's voice was awed as she looked around. "Even I've never been this far back before."

"Well, there's a first time for everything," Helen said with a grin. She sheathed her knife. "We should be fairly safe here – I've only seen one Saltriosaurus, and he was far enough away to outrun." She glanced over her shoulder as if to size up the other woman. "You do know what a Saltriosaurus is, right? Like an Allosaurus, but –"

"The European version. Yes, I know, dear. I did study archaeology at the best university on the moon."

"The moon?" Helen repeated. "Right. Fifty-second century. You'll have to forgive me; it's not often I meet someone from the future. Someone human, anyway." She grinned and gestured for River to follow her. "Come on. We need to find another anomaly. The Jurassic is really quite dull after a while, especially the Tithonian."

River obediently fell in behind Helen, still looking around in wonder as they walked. "So," she began, "what exactly are these anomalies?"

"Oh, you don't have those in your time? I guess the moon university didn't teach you everything, then," Helen said with a smirk. Her voice was soft and her footsteps softer; she moved through the forest with ease. "I'm surprised that the majority of humans still aren't clued in by then. Well, Dr Song, you saw your first anomaly just now. They're time portals – and space portals, too. Where exactly is the Stormcage Facility located?"

"In a mostly uninhabited sector of the Andromeda Galaxy," River replied. "Nasty little place. Always rains. Terrible for mobile phone reception."

Helen stopped in her tracks, turning her head to stare. "The Andromeda Galaxy?"

"Oh, you don't have those in your time?" River mocked. "We've been in Andromeda since the thirtieth century. I'm guessing you don't usually go planet-hopping through these anomalies of yours, then."

"No, I don't," Helen replied frankly. "I can't always get dates and exact locations in the future, but I'm fairly certain I've never been to a different galaxy."

River's eyes sparkled. "You're in for a treat, then. Tell me more about the anomalies." She brushed a stray curl out of her eyes.

"Well," Helen said, turning to continue leading them through the forest, "nobody's really sure what they are. I don't think there's ever been a period in time when the whole world has been aware of them – I think most governments of the world cover up their existence. Few people know what they are, let alone how to find or open them."

"But you do," River guessed.

"Yep." Helen grinned.

"How did you figure it out?"

"I cheated," was the casual reply. "I travelled to the future and stole their technology."

"So you're a thief," River said.

Helen paused again to look over her shoulder. "You," she said accusingly, "were in prison for murder."

"A misunderstanding." River waved a hand. "He and I both know what really went on."

"Right." Helen didn't sound convinced. As they resumed walking, she continued. "I only take what I need to survive. Clothes, food, tools. The detector is only for emergencies. It feels like cheating, otherwise."

"Cheating?"

Helen nodded. "It takes more than muscles or book smarts to survive in prehistory, Dr Song. It's a hostile environment where nothing will think twice about killing you. You have to be resourceful. You have to use your... instincts." A slow smile spread across her face. "By my century, human beings have become... lax. They've as good as lost all sense of smell and hearing; they're physically unfit... It's a lack of predators that prompts such laziness, of course, but that doesn't make it excusable. One day, a more successful species will rise and take our place, and then humanity will realise its mistake. Our reliance on technology has made us weak, Dr Song. Weakness in these times results in certain death."

She smirked over her shoulder at River, raising an eyebrow. "Are you still glad you chose to tag along?"

River Song folded her arms, smirking right back. "Why, Helen, from what you've said, the fun is just beginning."

"Your turn," Helen said in her soft voice, ducking deftly beneath a low-hanging branch. "You said you have time-travelling experience. How is that possible if you've never seen an anomaly before?"

River caught the branch as it swung back towards her face, clucking her tongue at the lack of manners. "That, my dear," she said, "is a long story."

"We've got all of time," Helen pointed out.

River laughed. "All right, then. When you put it that way... Let's just say the universe is a very different place in the fifty-second century. There are all kinds of ways to travel through time. Some lucky beings have the natural ability, but you mere humans have to use some form of technology."

"We mere humans?" Helen repeated.

River only smiled enigmatically. "Some devices are notoriously unreliable," she went on, "often with amusing or catastrophic side-effects. Others – mostly alien inventions – are a little more dependable. Emphasis on a little."

"How do you do it, then?"

"Oh, I've used my fair share of unreliable technology," River answered. "Time travel is a messy thing. If you aren't careful, you can create temporal paradoxes, rewrite history, or even collapse all of reality into a single point." She smiled cheerily. "It really is every bit as fun as it sounds."

"I know," Helen replied.

River tilted her head. "Brings a question to mind. How long have you been travelling through the anomalies?"

"I have no idea," Helen answered. "I'd guess it to be about six years, but I didn't keep track for a while in the beginning. It's not easy when you're moving in and out of time periods every other –"

She broke off, holding up a hand. River froze, her ears pricked for any sound, her eyes darting around as much as they could without her moving. "Listen," Helen whispered. River closed her eyes. The faintest gentle whispering brushed against her ears.

"An anomaly," River said. Helen pointed through the trees to where the occasional snatch of silver shimmer could be seen. The two women approached it slowly, both on the lookout for predators, but they saw none. "Do you know where it leads?" River asked.

"I haven't the faintest idea," Helen replied. "Shall we find out?"