Verkan Vall and behind him the entire Paratime Police organisation work to keep the other time lines safe from contamination while at the same time allowing resources to be extracted back to the home time line.

One of the time lines has a problem: a Venusian Nighthound has been taken back by someone who should have known better, to a time line in the 1940s. Worse, it's escaped to cause havoc amongst the farm stock. Verkan Vall has to drop onto that line and eliminate this terrible beast before its discovery corrupts the time line.