I can't find the edition that I read.  It was written to spur new interest with their youth, since the only way to be Zoroastrian is if both of your parents are. I'm not sure how much that's enforced though.  The first 3-5 chapters were free online to entice you to buy the full version.

Anyway, what I remember is that it starts with people living on the north pole, but it's summer all the time.  They're following rules from God/Maza, which is similar to rules in other cultures related to keeping the species healthy.  One that stands out for me is that each family is to have a sacred fire in their homes.

One day, a messenger from Mazda arrives and says, "The Dark One is coming, and when he does, you'll need to move from here, so start packing and getting ready now.  When it starts to snow, the Dark One has arrived, and it's time to head ... that way." and points toward Iran.  They claim they are the original Aryans, which is how the country got the name Iran.  Oh yeah, they claimed to all be of fair skin, blue eyes, blonde hair.

It starts to snow, everyone starts to caravan down.  The Dark One also brought the arrival of insects and cats.  I can't remember if that info was from the messenger, or if it's an observation of the members of the caravan.  While trekking south, a band of barbarians attacked them and took some of their women, and went back to Norway (my assumption).  The caravan settled in Iran, where it seems, everyone else was told to find "their land".  Meanwhile, it's peaceful in Lapland. :P