Chay fired. There was an explosive floomp noise, like a tennis gun.

A splash of horrific-looking white gunk wrapped itself around James's torso and right arm, gluing him to the nearest computer bank. The doomsday controller was knocked out of his hand and clattered onto the floor.

Chay stepped up - her enormous, white weapon sprouting a secondary barrel - and fired again, this time a tiny directional electromagnetic pulse, aimed at the controller. There was a satisfying electric snap and the controller exploded.

Then she was in James's face, one hand on his left shoulder, her gun morphing into yet a third shape and emitting a rising whine as it charged up. James stared down its inch-wide barrel, found himself utterly immobilised by glue and Chay's immense weight, and said politely, "Can I have a minute to explain?"

"You just blew up my home planet," said Chay. Wreckage was indeed spilling across the screen behind her. Frank, somewhere off to the left, flailed his arms and failed to say or do anything significant.

"That wasn't your home planet. If that was your home planet, it wouldn't have blown up," said James, looking Chay in what he hoped were her eyes. "History can't be changed. But there are two histories. And every time we go back in time, we also switch tracks."

Chay paused momentarily, then fractionally eased the pressure on James's shoulder. "Keep talking."

"In our timeline, there is no planet Tjörd. There's just an asteroid belt between Mars and Jupiter. It was always thought that the asteroids might be the remains of a failed or destroyed planet and now we know it was: it was blown up, here, now, by me. In the other timeline, your timeline, Earth was blown up and formed an asteroid belt, this time between Venus and Mars, leaving Tjörd behind for your species to evolve on. Everything's fine. Your timeline still exists. We can get back to it any time we want. All we have to do is go back in time again. Do you see?"

Chay thought about this for several seconds. "So you went back in time, and blew up Earth. Then you went back in time again, and blew up Tjörd."

"Thus creating both timelines, and answering the original question I set out to answer, which is what would stop me changing history if I tried. All that happened is I got bumped into a different timeline and created history instead of changing it. Are you getting this? Frank, did you get any of that?" asked James, rolling his eyes sideways to where Frank was standing.

"No, you're going to have to draw me a diagram," said Frank.

James looked back at Chay. She hesitated, but then backed up and released him. She pressed a button on her gun and the goop which had pinned James loosened itself, then leapt back into the barrel of the gun, as if yanked by invisible wires. The gun folded itself away and merged seamlessly with her pressure suit.

And James drew his diagram, and patiently explained everything over and over until Frank and Chay both got it.

"That leaves only one question," said Chay eventually. "How could you be so sure we were back in your timeline when you blew Tjörd up?"

"Because you were on this ship twice," said James. "You asked to be beamed up—"

"It's not 'beaming up'—"

"Whatever, whatever. But your computer didn't lock onto you. The flash of light we saw was from something else. Which meant there had to be two Chays on this ship, and your computer locked onto the wrong one."

"...Which means..."

"...You are going to be in this cavern-slash-spaceship-slash-time machine... again. In your future, but in the cavern's past," said James. "Which means somehow we're going to get back to where we started and send you around the loop a second time."

"How?"

"Permit me to demonstrate. First we need to return to the exact point in spacetime where we left," said James. He moved back over to his computer terminal. "As we're in the right timeline now, that's just a case of travelling forward in time four and a half billion years, by pressing this handy green button. May I?" he asked Chay.

Chay gestured assent.

James pressed the button. The screen above them became bare, steely wall once more. They had returned home.

"Finally," said Frank.

"Now all we need to do is to send you, Chay, back in time to the exact same place, time and timeline that we went back to when we started this crazy jaunt. Five minutes before the Earth was blown up, in your home timeline. Your pressure suit is still working, I assume?"

Chay checked some readouts on the back of her wrist, contorting her fingers oddly to do so. "Check."

"Now just go and stand in the middle of the cavern, out there roughly in the dark somewhere. It doesn't matter where - neither of us were looking when you appeared. I'm going to configure the time-travel conduits - "flux capacitors", to use the technical term - to project a smaller field around just you instead of this whole cavern. Now remember, when you get there, just do absolutely nothing until your computer brings you home."

"Just hide and keep quiet?"

"You got it."

"Right." Chay chose her spot in the middle of the cavern, and signalled that she was ready.

"Are you sure this is going to work?" asked Frank.

"Of course I am!" said James. "If it didn't work, it would change history, and history can't be changed. Have faith in causality, i.e., me."

Chay vanished.

***

Her ears popped painfully - they were similar enough to a Human's to do this - as all the air surrounding her pressure suit suddenly disappeared. For a few moments she tumbled in blackened space, making a conscious effort not to move or react in case she activated her suit's inertial controls and pushed herself away from the target point. She caught a good long look at the red, slowly collapsing dust cloud that was Earth, and a glimpse of brilliant yellow sunlight - then a makeshift spacetimeship clonked untidily into existence around her and she dropped a centimetre to the floor.

"What just happened?" said a voice.

***

There was a small thunderclap as air filled the hole in space Chay had left, but absolutely no light effect to accompany it. It being the first time he had witnessed a time-shift from outside, Frank felt rather let down by this.

The echo of the thunderclap died away. Frank wandered off to the corner with the vending machines in search of coffee. James turned his attention to his stricken doomsday device, and began to pick up the pieces one by one. "Am I still fired?" he asked.

"Well, you blew up two planets," Frank called back as the machine poured his drink. "I think that's against health and safety regulations."

"They were uninhabited at the time. And it turned out I was supposed to do it all along. You can't blame me for fulfilling my unavoidable destiny. I was a helpless agent of causality."

"I'm almost positive somebody tried that defence in court once."

"Did it work?"

"Don't think so."

"At least we proved my doomsday device work...ed. Also, aliens!"

Frank swallowed a sip of coffee. "Five minutes of one alien. Hello, kaboom, goodbye. Not even a photograph to show for it."

"Ah, they'll be back."

There was a pause while Frank drank and James collected small pieces of electronics from the floor. Eventually Frank found the presence of mind to ask, "When?"

"They should already be here," said James, glancing at his watch.

"Ah, you got me." Chay stepped out of the shadows again. "Ta-da."

Frank spluttered and began to choke on his drink.

"Work that one out," said James, to nobody in particular.

End