As we last left our heroes...

The alien looked at James and his offered hand.

"James..." said Frank behind him. "That's an alien."

"Seems likely," said James, lowering his hand but continuing to grin at the creature in front of them. "What, you think I might have just done something terribly insulting?"

"No, I just think it probably doesn't understand English."

"So? If we keep talking they'll figure out how to translate it sooner or later. Then they'll check their recordings and translate what we're saying right now and it would look really bad if it turned out we said something insulting during First Contact."

"Yeah, it might look really bad. How dreadful it would be if they decided, for example, to blow up our planet in retaliation."

"Can I get a word in edgeways?" asked the alien in perfect English. It spoke in a female voice.

"Hey, that's pretty neat," said James. "How'd you do that? Telepathy?"

The alien inhaled, about to launch into a lengthy, detailed and highly plausible scientific explanation, but thought better of it. "Yes."

"Good stuff. You have my name. This is my boss, Frank Lemnitz. You?" James asked the alien.

"I could tell you, but you wouldn't be able to hear it properly," said the alien. "If you have to call me something, you can call me Chay. What is your business in this solar system? I notice your craft has neither manoeuvring capabilities nor life support systems."

"You know how there's an asteroid belt roughly between Venus and Mars?" said James, who didn't know this for himself and was taking an educated guess.

"Yes," said Chay.

"We caused that asteroid belt, four and a half billion years ago," said James. "It was going to condense into a planet called Earth, which was going to become our home planet when we eventually evolved on it. But we went back in time and blew it up."

Chay nodded sagely. "Why?"

"I was trying to prove that history can't be changed," said James. "Interesting side note: turns out it can. And now we've come back to the present day and there's a completely different planet in our solar system, but no Earth."

"But that's impossible," said Chay. "Everybody knows history can't be changed. We get taught it in science lessons. Well, we have the information fed to us chemically during infancy, but it's the same thing."

"Exactly. That's what I said," said James. "You understand this, I understand this, it's just the universe which seems to disagree."

"Look," said Frank. "This is getting needlessly complicated. Can't we just go back in time and stop ourselves from blowing up the Earth?"

"That's your solution to everything, Frank."

"Seriously, though. Can we do it?"

"We can try," said James. "The only thing is, nobody stopped us from blowing Earth the first time around. And... well, I was going to say history can't be changed, but—"

"Maybe Earth's past history, its real past history, is the one where we try to destroy it in the past, but we - us-we - stop ourselves from doing it," said Frank, enthusiastically.

"That makes no sense, grammatically or otherwise, but I have no better ideas," said James. "It's got to be worth a shot."

"Will you let me transport myself off your... cave... first?" asked Chay, who had concluded she was in the presence of a pair of fools and was regretting ever getting involved.

"Be our guest."

Chay operated a control on her pressure suit and said "Ship, recall."

There was a flash of red-blue light - behind James and Frank. They whirled around. Chay's right arm unfolded rapidly into an unpleasant-looking projectile weapon trained on the place far back in the dark cavern where the flash of light had come and gone. There was nothing there. Something hadn't arrived - something had left.

Glorious comprehension dawned over James MacPherson's face. "Now I get it!" He turned back to his computer terminal and slammed a button.

The green planet on the gigantic screen changed. It became dusty, blurry, red-hot, with an accretion cloud just like young Earth had had. Frank, James and Chay all gazed at it.

"One last thing. What's your planet called?" James asked Chay. "I mean... what can we call it in our language?"

"Tjörd..." said Chay, although it would be some time before Frank or James figured out how to spell it.

"Great," said James, picked up the doomsday controller from the floor and - before anybody could do anything - pressed the trigger a second time.