4.5 stars.I really enjoyed this book. It had a great story, plenty of action and a number of twists and turns. I had a hard time putting this one down. Anna is the daughter of Arthur who runs the genetic research program for The Branch. The program consists of studying and testing 4 guys who have been living in their basement for the last 5 years. This is where the story starts and ultimately, takes a turn.The guys are all very different - Trev, Cas, Nick and Sam. Each one has a different relationship with Anna and without giving away the story, there is reason why. Anna is closest to Trev and Sam. Nick is pretty ornery and Cas seems to be the hungy, easygoing one. I honestly liked each and every one of the characters and am looking forward to learning more about each one. Their backgrounds are the key to understanding where they come from but I am also interested to see where they go from here.The dynamic between Anna and Sam is interesting. I don't want to give anything away regarding the story so just trust me when I say I thought it was going to be one thing and ended up with a bit of a twist. Ok - I know this was a pretty vague review so just trust me when I say that this was a great first book to the series. If you like a bit of mystery, a good story and something fast paced, you will enjoy this one. I am very much looking forward to the next book.