For decades, Dexter Morgan has been dealing with a dark secret—he’s not like everyone else. Sure, on the outside, he’s likable Dexter, with a doting girlfriend, a close relationship with his sister and a great job working as a blood-spatter analyst for the Miami police department. But, on the inside, he’s governed by the Dark Passenger, a sinister presence who dictates the true Dexter’s every move and helps him live out his real desire: killing people.

You see, from a very young age, Dexter had always felt different. He didn’t want to have a life, he wanted to take one. When he was a teenager, the adopted boy’s father, Harry, a police officer, noticed this difference in him. Instead of committing him to a psychiatric ward or locking him up, Harry trained Dexter how to harness and use his evil for good, teaching him to hunt and kill people who deserve to die. Serial killers, mostly. Now, years later, out of respect for Harry, and thanks to the benefits of working for the cops, Dexter is able to locate and kill Dade County’s undesirables.

Part of what has made the Dexter series fun is that he thinks he’s not human. He just acts how he thinks people are supposed to, holding a job, dating, talking about the weather, etc. His thoughts and ideas on human interactions are quite amusing, as if he’s an alien from another planet trying to blend in. And, in the first two books, the fact that no one has suspected him of being a killer has always been a strong point. It felt good to be in on the joke, as he seamlessly slid from steely killer to lovable doofus.

Unfortunately, Lindsay has deviated from the formula a bit in “Dexter in the Dark,” the third installment. Now, Dexter’s sister Debbie is on the secret, as well as his girlfriend’s delinquent children, themselves blooming psychopaths since suffering abuse at the hands of their father when they were very young.

Debbie is head detective at the station where Dexter works, and his keen insight into the criminal mind has always unwittingly helped her solve cases. But, at the end of the second book, Debbie learned of Dexter’s hobby, and now, in the third, she’s turned into a nag, pushing him to use his firsthand knowledge of sickos to help her solve a particularly gruesome case involving cult killings.

Only, Dexter can’t. Something about the details of the case has scared his Dark Passenger away, and he’s no longer talking to him. Dexter is left worrying that he’ll have to play out the rest of his days as a normal human. To make matters worse, his girlfriend Rita is pressuring him about their upcoming marriage. (Marriage being another one of those things Dexter has agreed to as a way of keeping up the “human” charade.) Between picking out a caterer and waiting for his evil inner voice to return, Dexter dodges Debbie and wrangles Rita’s kids, who want him to teach them his trade.

While Dexter’s internal monologues are still entertaining, the concept of a serial killer hunting serial killers has been stretched thin here. What made the other Dexter books interesting were the other stories happening around his hunting habits, and the story in “Dexter in the Dark” is not particularly interesting. What was once fresh and fun has gotten stale. The book has a strong start but soon loses its shine. Yes, they are in Miami, but does someone in every book have to get rid of a body by dumping it off a boat?

Author Jeff Lindsay pushes us even farther at the end of the book, asking us to believe in the supernatural, as if a serial killer killer weren’t unbelievable enough. Book three might be a good time for the series to experience Dexter’s favorite thing: death.