时间犯罪(Time Crime)简介：

These are the classic H. Beam Piper stories of the Paratime Police, a law enforcement organization which has learned how to move between parallel universes with alternate histories. They must also protect the secret of paratime travel.

In the Paratime universe, there are an infinite number of timelines, but in each timeline, events occurred differently. They are grouped into five Levels, based on the probabilities of success of an attempt by Martians to colonize Earth 75,000 to 100,000 years ago. They use this time-traveling ability to trade for goods and services which their own, exhausted Earth cannot provide.