Okay, so here's the thing. I think this book works really well if you think of it, and the entire series, as one long book as opposed to separate ones. The main characters from the last books Jared and Kelsey are back in this but not the main focus which falls to Thea and Marcus.
At the end of the first book, Jared and Kelsey have mated but have not yet found the letter telling them their lives are in danger from Marcus who came form another time. They find that letter at the beginning of this one. But it's more complicated because the Marcus from another time didn't just come from the future he had come from a parallel universe where things are quite different. However, ever since he showed up (and died in the first book) the people in the character's universe are having visions of their other selves. One of the things that happened in the other universe was that Thea and Marcus had a sexual relationship. In that case, neither of them were good people. Marcus because of his obsession with Kelsey and Thea because she never got over he love for Jared. That isn't the case here. The two clearly love each other but fight it because Marcus is a servant protector (not something she minds but he does) for the royal family sworn to stay away from romance because of his potential to harm a mate with his empathic abilities.
Also involved in this is Scott, Jared's best friend and general, who after an attempt to use the machine that moves time and also acts as a weapon, is captured by the United State Airforce. There, he meets and is interrogated by a linguist named Hope who is almost entirely blind from diabetes. In a dream, the two see visions of themselves in the other universe where they were husband and wife, she could see, and she was pregnant--something she can't do in this universe.
War breaks out because Jared's group and the other aliens on earth. We find out that the other aliens are stealing human bodies because theres were taken by a virus that nearly killed them. Scott is the son of two aliens who stole human bodies. This makes him angry and that is why he is fighting with Jared to protect the humans.
Marcus is also secretly the bastard son of Jared's father and therefore shares Thea's multi layered life having many forms although he isn't initially aware of this.
This book is sooo good. The whole series is totally different from anything I've read. Even Thea, who I didn't like in book 1, I loved here.
Great book. So far great series.