From audible.com:

Now that Pern can look forward to a future without the threat of Threadfall, the people are free to leave their protective stone holds and spread across more of the planet, as well as improve their lives with the newly discovered ancient technology. Not everyone is happy, though. Some resist the change, and consider anything new to be an "abomination". And the dragonriders are uncertain: without Thread, what will their purpose be in Pernese society?

Then a new danger - again from the skies - looms. Once again, the people must pull together...And turn to the only ones who can solve the crisis: the dragonriders of Pern!