Anne McCaffrey draws the reader back in time to an earlier Pern to tell the story of one of Pern’s great and true heroines.
An air of pleasant anticipation hung so thickly over the Halls, Holds and Weyrs of Pern that it had affected even the businesslike ways of Moreta, the Weyrwoman of Fort Weyr, where her dragon, Queen Orlith, would soon clutch. 
Then without warning, a runnerbeast fell ill. Soon myriads of holders, craftsmen, and dragonriders were dying; and the mysterious ailment had spread to all but the most inaccessible holds. Pern was in mortal danger. For, if dragonriders did not rise to char Thread, the parasite would devour any and all organic life it encountered.
The future of the planet rested in the hands of Moreta and the other deicated, lelfless Pernese leaders. But of all their problems, the most difficult to overcome was time. . . .