The story is of two factions of time travelers. The Tesla Institute and The Hollows. Both travel back and forth in time gathering important objects. We get the story through two "Rifters" Lex and Ember.
When Lex's girlfriend is killed during a mission he vows to change back time to save her. Lex and Ember must work together to piece their memories together and find out who they were before they were time traveling. I found this most interesting on how they figured it out.

I also enjoyed the two different approaches each faction uses to travel. I am looking forward to reading more of this series as it goes on.

A dark past full of secrets with no certainty of future events, this story is an absolute steampunk masterpiece!

I was really looking forward to this, but unfortunately it seemed to me like an exercise in How Not to Write a Time Travel Novel. The settings and times all run together and the character voices are annoying and all sound the same, which makes it very difficult to follow what's going on or whose POV is being presented (and which is especially strange considering this book was co-written by two different people). The pseudoscience makes zero sense even in a fantasy context. There's a big "twist" that was obvious fairly early on -- and which seems even more obvious when one considers the series' title. That was more or less the point when I gave up.