Thousands of years ago, society began to collapse on the the planet Arbre. The intellectuals of the time retreated into concents - monasteries in which the occupants are sealed off from the rest of the world - sometimes for centuries at a time.
Although the concents were founded to preserve the knowledge of Arbre, most of the occupants (known as avouts) are kept ignorant and even forbidden from learning many things about the world around them.
Erasmas - an avout of the Concent of Saunt Edhar - has his world turned upside down when his friend and mentor Orolo discovers an alien spacecraft orbiting Arbre. Orolo is thrown out of the Concent for daring to use forbidden technology that allowed him to observe the spacecraft. Shortly afterward, Erasmus and his friends are also exiled and pursue Orolo to discover what he has learned.
The novel climaxes in a battle between the Arbrelings and one faction of the aliens.
It takes a while to get there as it explores the concept of how alternate versions of the same reality can manifest themselves in multiple cosmos and what happens when people move between these alternate realities.
Anathem satirizes the idea of those who try to preserve knowledge, but restrict access to new information and discovery. The novel succeeds brilliantly with its social satire and its attention to detail. For some, it may fall short with its slow pacing and lack of action.
If you prefer your science fiction more cerebral than swashbuckling, Anathem is for you.