The departure lounge at Trans Time Tours was crowded with people waiting for their shuttles into the realms of history. Whitby goths mingled with sans-culottes, while Roman citizens sat alongside American Civil War camp followers. Time tourism was this year's 'must do' item, and everybody seemed to have it on their bucket lists.

Of course, there were numerous rules and regulations governing time travel. Most important of these was that the tourists could only go to the past. When people had tried to go to the future, they had been returned looking rather bemused and with a note reading, "Don't do it again, bub!" or something similar The people of the future made it perfectly clear that they did not tolerate their lives being disturbed by what they regarded as uncultured primitives. On the other hand, they seemed fine with people going back in time.

The second most important rule was that you did not interfere with the past. This rule was surprisingly easy to keep, and any returning visitors who complained that the Empire State Building shouldn't be there or that the prime minister had changed were obviously deluded or just looking for some kind of compensation. After that, the regulations became increasingly mathemical and abstruse, and were left to the engineers who controlled the time machines.

Of course, tight security was necessary. The UN had recognised that people had a right to go when they pleased (subject to the inscrutable whims of their descendants), but they also had to maintain a proper history. Imagine the devastation a zealot with a bomb could wreak at just the right moment in time! So, the people in the departure lounge were kept under close scrutiny all the time they were there.

Joe Mason was on duty when he heard a commotion at the departure gates. He hurried over to Gate 31, ready to intervene and restore order. There was a group of tourists, dressed in animal skins and carrying stone axes. They were protesting loudly to the gate guard and waving their weapons in a manner reminiscent of Fred Flintstone.

The gate guard waved at Joe. "Thank goodness you're here," he said. "I keep telling these people that their shuttle isn't ready yet, but they won't listen to me. They insist they're going to board now."

Joe nodded. "Let me talk to them." He turned to the leader of the group, an elderly woman who was clutching her deerskin wrap to her shoulder. "Look," he said, trying to sound reasonable. "You will have to wait."

The woman poked him with her staff. "Young man, we will board our shuttle now."

"But it's too soon. They need to prep the cabin."

The woman shook her head. "But we've been waiting for ages."

"Of course," Joe said. He tried to suppress a smile. "You're Homo sapiens, not Early Man."