In the not-very-distant future, aliens calling themselves The Atoners contact humanity. Millenia ago, they wronged humanity--and now they want humanity to know about it. They choose a few dozen people to travel to colonies of humans the Atoners established around the universe, and "Witness." What the "Witnesses" are supposed to see or do is left up to them--they are told that they'll know it when they see it.
By the end of the first third, both the reader and the characters have discovered what the Atoners have to atone for. The remainder of the book is about how the Witnesses--and the rest of humanity--deal with this revelation.
The characters are distinct, and complex, although they lack depth. I liked Cam a great deal, and grew to appreciate Soledad. By the end of the story, I hated Lucca, not least because the narration is so non-committal about him. As it was, I couldn't tell whether Kress knew one of her main characters was a privileged, patronizing prick.
I liked this story, but I was frustrated because Kress can do so much better. In her Sleeper trilogy, the consequences of a simple genetic manipulation on a tiny percentage of people are far-reaching, dramatic, and eminently believable. In this, a huge revelation has no impact on day-to-day life on Earth. Nor does that revelation have any affect on the other human planets. I really wanted to see the difference between societies! Kress is excellent at bio-ethics, but I wish she'd taken this story a little further.