This book by Anne McCaffrey is about a sickness that begins to spread through the planet of Pern. Everywhere, dragonriders, holders, harpers, and healers alike are all dying. Soon the truth behind the epidemic is revealed, and it is up to the dragonriders, who can go anywhere, anywhen on Pern in 8 seconds, to distribute the finally found vaccine. The dangers are great; the dragonriders are needed to fight the Thread, a parasite that falls on Pern and eats every organic thing in its path. Moreta and other selfless Weyrleaders must find a way to distribute the vaccine to everyone on Pern in the same day. Of every problem, the challenge they had to overcome was time. But the heroic deeds of Moreta end in tragedy; a tragedy destined to change Pern in generations to come.