A SpeculativeFiction novel by Creator/NancyKress.
An alien race calling itself the Atoners has put up a website and then landed on the moon. No human has seen them -- they have only heard their voices. The Atoners claim to have committed a crime against humanity, and they recruited twenty-one Witnesses from the human race to see what exactly they've done. The Witnesses were taken to seven systems of two paired life-supporting planets; they were not told what to look for, only that they would know it when they saw it.
The first part of this novel focuses on three witnesses in particular:
* Lucca, an Italian who is also British by marriage. He is a trained anthropologist and a widower who has not yet gotten over his loss. He is also a hard-core materialist -- he very definitely does not believe in the afterlife. So, he crashlands onto the planet he's witnessing on, breaks a leg in the process, which limits how much he can investigate; that this society has no visible structure but gets along anyway is one of the most interesting things he notices -- that, and the euthanasia he witnesses on the way to the lodge he stays at. One of the children befriends him, which is a mixed blessing, since she and her ImaginaryFriend seem to know way too much about him...
* Cam, who is a ManicPixieDreamGirl, very childish and resistant to authority, and possibly a CanonSue. In her interview, she breaks all the guidelines the US military gave her, including a direct request not to ask what the Atoners are atoning for; she is selected anyway. She goes to a planet in which ''everything'' in every culture is guided by the great game of "kulith." She is met by a society that is cruel and brutal and at war; they try to kill her, but the Atoners have given her a forcefield that protects her from practically everything. So they send Aveo, a scholar who had been declared a traitor, to speak with her. He assumes she's a foreigner, and so her TranslatorMicrobes are initially set only to the language he used on her, and not the one of the people she's dealing with. He teaches her kulith and tries to show her how his society works; she breaks the Prime Directive as soon as she knows ''anything'' about how the society works.
* Soledad, who is left in orbit of the two planets to record and observe. She can see little directly, but hears everything. Lucca talks to her a lot until his young friend asks what a "soledad" is (his attempts to keep his communiques private failed). Cam gets Soledad to move her craft after she kills the lead soldier holding her back, unworried about Aveo and the concubine she "rescued" because they don't speak English and gets ''critical'' info from her.
Part two is what happens about three years later, after everyone gets back with the news of what the Atoners had done. All the surviving witnesses are famous. There are six people who are spreading the word on what they have done, including Cam, who is spreading it with an extremely high profile. Lucca also saw it, but since he didn't ''believe'' in what he saw and since he went into seclusion almost immediately after returning to Earth, he usually isn't counted. Soledad, who didn't see it but knows what was seen, sort of, is also trying to live a quiet life -- she's effectively in witness protection -- but her relatives, whom she has always hated, prevent that dramatically. The Atoners themselves have gone silent and don't appear to be doing anything, the governments of the world aren't doing much to help, and several groups are trying to ''kill'' Witnesses. In this environment, another of The Six, Frank, decides that the problem ''can'' be fixed and that God wants him to fix it (it's official penance) -- and then finds he needs to get help doing it. Between him and Soledad, chaos ensues.
!! Tropes in this work:
* FirstContact: Occurs when the Atoners reveal themselves.
* ImaginaryFriend: The child who befriends Lucca has one.
* TranslatorMicrobes: Given to the witnesses when they're sent to other worlds.