Humans inadvertently introduce catastrophic changes to an alien species near one of their colonies. Avice Benner Cho is a hyperspace pilot who comes back to her childhood home as an adult. She grew up in Embassytown, a colony of Bremen. Embassytown is on the border planet of Arieka, which exists at the edge of the known Universe and its location makes it strategically important.
On Arieka several species coexists, mainly non-indigenous aliens and the humans, but also a local Ariekei species, commonly called the Hosts. The Hosts have two speaking orifices and speak by producing two words at the same time, so their language is very hard to reproduce.
More importantly, the Hosts are purely literal minded, their language can only refer to the world as it is. They can't lie or speak metaphorically. They also can only understand other beings when their speech is also produced by two voices but one mind. Single humans, or machine reproduced speech is literally incomprehensible to them and they don't see single humans as people.
The humans have developed specialist pairs of twin clones, called Ambassadors, who act as a single individual with neural connections so they can think as one person and speak in perfect harmony. The hosts understand these as ‘real' sentient people and this allows the humans to establish a trade and acquire the sophisticated Host biotechnology.
As a child Avice was recruited to perform a service for the Hosts. They sometimes stage very specific scenarios and act them out so that they have a simile/expression to refer to in their language, since they can't just make up imaginary scenarios. Avice acts out and becomes part of their language, a great honor.
As and adult Avice returns home with her husband Scile, an off-worlder linguist who is fascinated by the Language of the Hosts. Her marriage is friendly, but they are sexually incompatible and Avice ends up in a relationship with an artificial intelligence named Ehrsul. Between her status as a traveler through hyperspace and her connections, Avice sees a lot of the politics of Embassytown, and the tensions between the local colonists who have relatively little influence and the colonial powers sent from Bremer.
In an attempt at reinforcing their off-world colonial power, the Bremer authorities have sent a new Ambassador called EzRa. He can speak to the Hosts but is composed of two linked non-cloned individuals instead of the usual engineered twins. Ezra's strangeness causes a dissonance in the minds of the Hosts, who find themselves addicted to listening to him talk, to the point that they forget to even eat.
Scile is fascinated by the language and the Hosts, and Avice takes him to see the Festival of Lies. A taboo and underground event where some Hosts attempt to expand their thinking and language by speaking untruths. They do this by having an Ambassador state an untruth and then they attempt to repeat it, though so far only one Host is apple to speak outright lies, and is greatly admired for it.
They also meet other humans like her who have become part of the Host language. Scile is horrified at the attempts to lie by the Hosts, believing it a corruption of their purity and a form of evil. He becomes involved in a plot to assassinate the Host that can successfully lie.
Meanwhile as more Hosts become infected, things start falling apart. The humans rely on the Hosts for basic necessities and find themselves coercing the addicts by controlling their access to their language drug. The Ambassador becomes power hungry, and the balance of power shifts.
Finally some Hosts realize that their society could collapse if the effect continues spreading, and for the first time they turn violent against the colonists. They deafen themselves so they can't hear the Ambassador, freeing themselves of addiction, but becoming Languageless (and therefore less than sentient in the eyes of Host society). There is an attack of Embassytown, and violent thrashes within the Host society.
As the effect spreads through the entire Host society and all of its bioengineering begins to break down. The Languageless faction grows and organizes in an attempt to wipe out the Infected and the source of the infection, in hopes that the next generation will be free of the madness.
Avice realizes that something has to be done or the Hosts and the colony alike are doomed. She manages to form an alliance of an Ambassador, some Lying Hosts, and others. The group goes to confront the Languageless that are marching on the city.
She realizes that the attempts by the Liars were a way of expanding the Host limitations and achieving true sentience. Together the group uses this knowledge and successfully manages to break through the preconceptions and the addiction and convey several critical new thoughts; that single beings are sentient, and that language and sentience are not synonymous.
Ultimately the Hosts and the colony are changed forever, as the Ambassador's stranglehold on access to the Hosts is broken and the Hosts are able to communicate with single humans and other aliens. The infection is shaken off as the Hosts get past the concept of non-literal language, although some of the Languageless form separate enclaves to raise their young in the old ways. The process has essentially released the Hosts from the previous semi-sentient purely literal state, and they have emerged into full consciousness.
After everything is over, Embassytown is set to become something bigger than it was before, with Hosts and others coexisting in a very different relationship. Avice decides she will go travelling again, this time outwards beyond known space.
Best part of story, including ending:
The central ideas around language, the ability to conceive things that are not already known, and the grasp of intelligences outside ourselves is really meaty stuff. That a being can be intelligent but not sentient is expressed in this book as well as I've ever seen.
Best scene in story:
The Festival of Lies is fascinating, it's incredibly frustrating to see the Hosts attempt to get their heads around what is a basic concept for a human but is literally inconceivable for them, but it's also really interesting to conceive of non-truth as being at the heart of sentience.
Opinion about the main character:
Avice is someone who is at the heart of several important events without being special or unique in a cliched manner, which is nice, she is a bit of an observer rather than a doer, though and a bit harsh on her homeworld after being away from it.