Third Shift: Pact is the last of three novellas in the middle volume of Hugh Howey's Silo series. (It's also the point at which talking about the series gets much easier; the last installment, Dust, is straightforwardly a single, full-length novel.) Pact completes the flashback-driven middle of the Silo arc, and is the strongest of the Shift stories.
As with the two previous Shift stories, this novella alternates between two perspectives. In one, a protagonist is again awakened from cryosleep to oversee the response to a crisis. This time, some unknown hacker has actually swapped his identity with someone else; it's not he who was meant to be awakened. With all the personnel surrounding him oblivious to the change, our hero is free for the first time to seek answers to his own questions about the silo.
This is the arc that really puts the entire story on an inexorable road to completion. But it's a more nuanced tale than a simple do-gooder striving for answers. Even as our hero is digging to get to the bottom of the silo's mysteries, he's descending into moral ambiguity. Or even outright moral darkness. Not only is part of him starting to subscribe to the very system he's trying to expose, but the methods he uses in his investigation are decidedly not the actions of a hero. It adds an interesting texture to the overall tale, because even though "the other side" has always had a point of view in these books, it hasn't necessarily been a sensible one. Yet the scales are now balancing a bit, not necessarily because the opposing arguments become more clear, but because our hero is sinking to their level.
The second story line doesn't contain any surprises in terms of plot, but it is perhaps the most fascinating character study of the series. Back in Wool, we got to meet the character of "Solo," an odd but sympathetic man who grew up in isolation. Now, Pact fleshes out his back story, showing us exactly what his experiences were like. It's a quite personal depiction of the human costs that are sometimes abstracted as this post-apocalyptic story often takes a larger view.