Rukbat, in the Sagittarian sector, was a golden G-type star. It had five planets, two asteroid belts, and a stray planet it had attracted and held in recent millennia. When men first settled on Rukbat's third planet and called it Pern, they had taken little notice of the stranger planet, swinging around its adopted primary in a wildly erratic orbit -- until the desperate path of the wanderer brought it close to its stepsister at perihelion.

When such aspects were harmonious, and not distorted by conjunctions with other planets in the system, the wanderer brought in a life form that sought to bridge the space gap to the more temperate and hospitable planet.

The initial losses the colonists suffered from the voracious mycorrhizoid organism that fell on them were staggering. They had divorced themselves from their home planet, Earth, and had already cannibalized the colony ships, the Yokohama, the Bahrain, and the Buenos Aires, so they would have to improvise with what they had. Their first need was an aerial defense against the Thread, as they named this menace. Using highly sophisticated bioengineering techniques, they developed a specialized variant of a Pernese life form which had two unusual, and useful, characteristics: the so-called fire-lizards could digest a phosphine-bearing rock in one of their two stomachs and, belching forth the resultant gas, create a fiery breath which reduced Thread to harmless char; and they were able both to teleport and to share an empathy that allowed limited understanding with humans. The bioengineered "dragons" -- so called because they resembled the Earth's mythical creatures -- were paired at hatching with an empathic human, forming a symbiotic relationship of unusual depth and mutual respect.

The colonists moved to the Northern Continent to seek shelter from the insidious Thread in the cave systems, new homes that they called "Holds." The dragons and their riders came, too, housing themselves in old volcanic craters: Weyrs.

The First Pass of Thread lasted nearly fifty years, and what scientific information the colonists were able to gather indicated that Thread would be a cyclic problem, occurring every 250 years as the path of the wanderer once again approached Pern.

During this interval, the dragons multiplied and each successive generation became a little larger than the last, although optimum level would take many, many more generations to reach. And the humans spread out across the Northern Continent, creating holds to live in, and halls in which to train young people in skills and professions. Sometimes folks even forgot that they lived on a threatened planet.

However, in both Holds and Weyrs, there were masses of reports, journals, maps, and charts to remind the Lords and Weyrleaders of the problem; and much advice to assist their descendants when next the rogue planet approached Pern and how to prepare for the incursion.

This is what happened 257 years later.