Sometime in our future, the people of Earth go forth and colonize other planets. One of these is the planet which came to be called Pern.
Over the years the people of Pern created their own legends and mythologies, and the home planet of Earth, and the rest of their ancient past, was forgotten. The star ships they arrived on were cannibalised, mainly for mining purposes, and eventually there was nothing left - so these were forgotten too, and no reminders of the home planet remained. Pern was like Earth in many ways, but low on many of the raw materials needed for technological advance, so they stayed perpetually in the equivalent of Earth's Middle Ages.
There was a danger every 200 years or so from a desolate nearby planet - known to the locals as the "Red Star". From these life forms known as "Threads" fell upon Pern, seeking to reach the vegetation. The Threads sucked the life force out of other living things, thus rendering barren anywhere they fell and managed to get a hold. The whole Southern continent of Pern had been ravaged in this way.
The people of Pern had bred a new life form - called Dragons due to their similarity to the ancient mythical beast - to combat this menace. The dragon, by chewing Firestone, could breath fire and incinerate the Threads before they reached the ground. The Dragons had riders, who came to have a high position in Pern, and lived in separate areas called "Weyrs".
The Red star does not always pass close enough in its orbit for the Threads to make the jump across to Pern. The last time it happened, as this story begins, is nearly 400 years ago. Thus, the Holds (where the normal folk live - Pern is divided into three subsets - the Weyrs, with the Dragons and Dragonfolk, the Holds, where the average person lives under a local Lord, and the Crafthalls, which is where the masters and pupils of the various crafts of Pern live and work) have rather forgotten their debt to the Dragonriders. Many openly disbelieve the stories of the menace of the Threads. Even some of the Dragonmen, including their current leader, doubt if the Threads really exist. But the signs indicate that the Red Star is nearly in position to pelt Pern with its destructive cargo. And some Dragonmen still do believe fervently…
There is now only one Weyr left, and one Queen Dragon, who is dying. With a single Queen egg about to hatch, there is a Search for a suitable Weyrwoman in Impress it. (Impressing is the process of the new hatchling attaching itself to a young person - a boy unless the Dragon will be a Queen). F'lar finds a girl called Lessa in Ruatha Hold, who along with him believes, although not quite knowing why, that the Threads are real and that they're coming. But neither of them could possibly see the way events are about to unfold about them, or just how vital they are to the very survival of Pern…
The above-mentioned F'lar and Lessa, despite being the main heroes of the story, are far from perfect characters. F'lar is arrogant and Lessa vengeful and short-tempered. But you still care about them a lot, F'lar particularly being a very endearing character, once you realise the reasons for his actions and attitudes. The characters are all well drawn, have histories, react realistically to situations and events, and develop as the story goes on. The main focus of the book is, I felt, the relationships between the different characters. Even the "bad" characters, with one notable exception, have their good points and reasonable reasons for what they're doing.
The most interesting aspect is the relationship between the Dragons and their Riders. Yes, the Dragons have distinct personalities, and can communicate with their riders through telepathy. The bond between them is explained extremely well - shown rather than explained, in fact.
Thankfully, both males and females are portrayed as good and bad in turn, with the men portrayed as 3-dimensional characters who actually havngs. (Sorry but from experience many female writers seem to opt for a "Women good, men bad and emotionally repressed" approach, which often leaves me feeling that they are pouring out their feelings of a bad relationship into a book without letting the story get in the way. Of course, many male writers also fall into a similar trap, while others of either gender put too much emphasis on the action and fail to develop the characters properly. McCaffrey gets the balance exactly right, however, with character-development the main theme without letting the action scenes slip or having too many or too long gaps between the action.
There are some great characters that only come into play later in the book, too, which add an extra bit of spice to the proceedings. I particularly liked Robinton (the Master Harper) and Farrandel (the Master Blacksmith), though I won't tell you anything more about them to avoid spoiling any of the story if you haven't read it.