Lord Kalvan of Otherwhen — H. Beam Piper

The Paratime Police patrolled the vast number of alternate time-dimensions. Their aim was to keep the existence of the alternate Earths a secret and prevent these Earths from mixing and destroying each other.

But the Time Police made mistakes sometimes, and they made a big one when a seemingly ordinary Pennsylvania State Trooper named Calvin Morrison, from the Fourth Level, Europo-American, Hispano-Columbian subsector, was accidently switched into the Aryan Transpacific sector, Styphon's House subsector.

In just a few weeks, Morrison was being hailed as Lord Kalvan, and was masterminding a campaign that could blow the whole Paratime secret sky-high!