Shift gives us the origins of the silos and the events that led to humanity's downfall. The book alternates between different timelines, showing the construction of the silos and the early days of the silo's inhabitants. 

The premise of Shift is intriguing but it felt a little long and reading it felt like a formality. I might appreciate it more on reread, but jumping to a different timeline with different characters was an interesting choice for the middle book of a trilogy.