From the way Dexter refers to the Dark Passenger in earlier books it's almost as if he's a seperate entity. This takes that idea and creates a back-story for his Dark Passenger. In this story there are bodies turning up, burnt and headless and when Dexter goes to the first crime scene he loses his companion. Finding himself without insight into the crimes and feeling like he's lost a part of himself he has to find out the whodunnit and the why of the disappearance.
Add to that the continued development of Rita's two children as their own special monsters and horrors of horrors a wedding to organise, his own.
It just didn't flow like the earlier books, it stuttered through what could have been an interesting concept, but as a continuation of the Dexter series it just didn't seem to fit. The ideas of who and what the Dark Passenger is were interesting but just fell a little flat in the execution.
