Alyssa and Sam had an affair - blistering, exciting and could have been so much more - until Sam got married to the mother of his child. Fast forward eighteen months later, and Sam finds his wife's dead body - and his daughter missing. Alyssa, against her instincts decides to help Sam find his baby girl - and nothing more.
It might be best if you read the books before this one. It doesn't really stand up alone as a one off, and is probably more a 'bridge' for the rest of the series. I say this due to the relationship of Sam/Ayssa/Gina/Max - there's a LOT of back story there, and the author assumes that you know it - to the point where I wanted to put down the book and read the others before this one, so I could understand what was going on so much more.
Also, there is a lot of time shifts, from Sam in the present, flashbacks to various stages of Sam's childhood- to events that happened before he was born. Brockmann uses these events to paint a deeply complicated background of Sam Starrett that one appreciates - but after a while the time shifts became annoying and drew me out of the story.
A strength of the book was the pace of it - there was always something happening, something going on. You do have to pay attention while reading it, so that you can relish the payoff in the end. Alyssa and Sam aren't sweet characters, no- they are both very strong personalities who've hurt each other in the past, so there's caution to their interactions.
Overall, a decent read. I wouldn't go out of my way to read it again, and if it were one of the books that you bought in an airport, you'd feel rather disgruntled on the plane. Oh well.
ETA: read it again, especially after reading "Over the Edge" - yeah, I truly get Sam/Alyssa's relationship now.