The premise of the book is spectacular and the exploration of its far ranging effects on humanity is so very well thought out. Alternate earths become accessible to humanity via stepping boxes. These alternate earths were never developed by humans. They were instead developed by evolution in a myriad of fascinating ways. The difference from earth to earth is not about the people in them (there are far and few), they are about the kind of creatures that managed to evolve there with only slight modifications to those moments in history when evolution took a leap. Bipedal lizards, massive sea creatures, sentient bacteria. There are versions of earth that were affected differently by various cosmological events including a world with no moon. The possibilities are endless. 
 The story follows Joshua Valiente and a AI unit called Lobsang as they explore the millions of earths in search of answers. Just how many earths are there? Are there other human civilizations in them? How did we not know about them before? What is causing Joshua's head to feel so much pressure as the travel farther and farther out from original earth? There are some side plots involving police officer Monica Jansen, but honestly I feel like her name came up a lot, but I couldn't tell you much else about her. While I enjoyed everything about the premise of the book, I found the characters very dry and their interactions and personalities very unnatural. This book held its greatest strengths when explaining its concept rather than when it had to deal with its characters or their actions.