Kala and the Doctor both hated each other by now, but they were each too busy surviving to be showing it. They raced together down through the maze of caverns, not daring to look back in case an oil creature was just behind.
However proud a race the Time Lords were, their constitution was no match for that of a fully trained Earthling astronaut. The Doctor was gasping for breath by the time she'd skidded to a halt, while Kala looked like she could keep on running for miles.
"We're far enough down now," said the Doctor as she panted and leant against the cavern wall. "Any oil that follows us'll just collapse to nothing. So we should be safe, at least for now."
"You're not safe, while I'm around," said Kala. "And I don't believe that we're so safe from you."
The Doctor did something with her torch that made her whole expression alight, so it was clear how tired and angry she really was. Yet that tiredness didn't extend to her eyes, which still stayed soft even after everything Kala had said.
"Look," she said. "I know you don't trust me. And that's fair; it's not like I've given you anything much to trust in. But I've seen every fear there is in the time I've been alive. And I know that fear's what's really driving you now. You think it's in danger, don't you? All of your civilisation. You think if you don't succeed, that it's possible your whole race will die."
"You don't know a damn," said Kala, and someone who wasn't the Doctor might have missed the small catch in her voice.
"You might be surprised. My planet burned, or I thought that it had. So I know what that dread is like. To worry that your whole world is ending— not in a story, but really, and how all the stories you ever heard don't prepare you for that at all. I've seen so much; been crushed in so many ways. But I still wasn't ready, when I first felt that weight in my hearts."
Kala's shoulders softened, despite herself. But her skepticism stood firm in all of the rest of her body.
"Wherever you're from," she said, "things can't be as bad as they are on the Earth, not if you can travel this far. Whatever your story, you don't know what it's like when it happens to us."
"No. But I know a lot about your planet and its people. Not because of documentaries. Because I cherish it, even love it. I know what it means for it to end."
Kala snorted. "Surely we'd have seen someone as ridiculous as you if you'd come down…"
She stopped herself, and the Doctor could see her reorienting within her mind. When Kala next spoke, it was in a tone that far harder than any she'd yet used upon the asteroid.
"This is how you do it, isn't it?" she said. "You find a way in like a burglar, and before long people think you're their greatest friend. What did you call that thing up there, when it was dragging my real friend to death?"
"An Inverine. It's called an Inverine. But it's not"—
"Whatever its name really is, it's not the most dangerous of you all. It killed Drocks, but that's nothing next to you. You'd overthrow the Earth if you got half the chance, wouldn't you? And you'd probably have us cheering when you did."
"That's not what I am," said her foe. "It's not what the Doctor ever is."
"I know exactly what you are. You're all Humans, aren't you? The Inverine is a Human, and you are too. Your friend is a very little Human, who probably isn't even a child. You've sent in your best so our mission never succeed. So that Earthlings will never go on to reach the stars."
"None of that's true," said the Doctor.
Kala glared at her. "And everything else you've told us is?"
The Doctor couldn't meet her gaze. "No," she said, with regret that sounded genuine.
"Then why exactly should I be trusting you now?"
The Doctor thought very quickly indeed.
"It wasn't breathing," she said.
"What?"
"The Inverine. It wasn't breathing when we saw it. Or did you not notice that? Me and Chris both need our spacesuits, but it was just standing exposed."
"For all I know there's no air on Hume and none of you need to breathe. All I know for sure is that there's something you're not telling me."
"I could kill you, though," said the Doctor, "if I was what you think I should be. Bash your helmet in with a rock and wait 'till you drowned in raw space. No one would ever know."
Kala snorted. "You think you could kill me? I eat mammals like you for breakfast. Literally."
"Oh, those mammals won't have been quite like me," said the Doctor with a smile. "These days I'm pretty used to being underestimated. Not usually for this reason, granted, but still…"
She trailed off, and sighed.
"We shouldn't be arguing about which of us can kill the other," she said. "Not when both of us are in real danger."
"You should worry more about your friends. You might not like to think about killing, but us astronauts have to. We are all trained for it, if it's something that has to be done. Now, I've never done it, and I don't want to start— but I will kill the three of you if we come under attack again."
"There aren't three of us. There's just me and Chris."
A tiny expression of surprise twitched on Kala's mouth.
"That was a test, wasn't it?" said the Doctor.
"The fact you noticed means I'm much less likely to trust you."
"I don't think there's anything I can do to make you trust me," said the Doctor. "You've already decided what I am."
"You're a monster. But perhaps there are more of those here than you think. I'm a long way from the worst of us, Doctor," said Kala. "We need to find Bol, and you should hope he's done nothing to your friend. He might seem pleasant on the surface. But even you can't always tell what's happening underneath."