Far in the future …

"General, sir, I've detected a gigantic spacecraft appearing just around the far side of the moon."

"Incredible! Send out a UE Space Defense Force squadron to intercept … Uh, I mean, greet them."

"General, this is Captain Smith of Intercept Squadron Alpha One. We were invited and boarded the ship. Extraordinarily, they appear to be humans."

"We are humans, Captain."

"But from where?"

"Not where, Captain, when. We're from your future, many centuries from this present."

"Why are you visiting us now?" asked the general over holo-communications.

"I'll explain. Please follow me, Captain."

The time traveler escorted the captain to an enormous cargo bay with rows of stacked small cages, each about one square meter in size.

"Why all the cages?" asked the captain.

"Here's our problem: In the future, we're a depressed people. We have one another for companionship but none other. Our historical records show that pets would provide us the necessary therapy and unconditional love."

"Does something happen to cats and dogs in the future?" asked the general.

"No, General, nearly all of the world's lifeforms go extinct with only a handful of survivors. So we calculated that if we rescue pets, it won't create a temporal paradox."

"But how does it happen?" asked the captain.

"You haven't detected it yet—an unstoppable killer asteroid is heading your way."