A small town detective is forced into an inconceivable mystery when a severed hand, genetically identical to his, appears next to the headless body of a boy. When the case leads back to his own missing son, he’s introduced to a world of parallel universes in which he may be able to save his family and the world.

Just a possible version to consider (although I'm not sure it is what you intended) -
When a small town detective is faced with a gory trail of evidence that implicates himself in the fate of his own missing son, he discovers a series of parallel universes which he must navigate in order to save his family and the world.

As I understand it, the hook, the central concept of the story is the idea of parallel universes.? And father's goal seems to be to save his son (and family?) in an alternate universe because he was unable to do so in this one.

Well, that could be a dramatically interesting story. But, as your logline says, you are proposing a story that is? an?"inconceivable mystery"? -- that is, it's unbelievable????It?requires the suspension of normal?logic and of the?scientific rules that govern?the world as we know it.

Scripts about parallel universes (or time travel)?can be very?hard to write in a way that will make them believable.?? And it can be even harder to write a logline?in 30 words or less that accomplishes the same feat.

I presume the "strange world" to be the parallel universe.?There seems to be a? cause-and-effect link between the severed hand and the headless body that ?leads to the discovery of a parallel world.? But I have no idea what it is.? How does one lead to the other?

As I understand it, the hook, the central concept of the story is the idea of parallel universes.? And father's goal seems to be to save his son (and family?) in an alternate universe because he was unable to do so in this one.

Well, that could be a dramatically interesting story. But, as your logline says, you are proposing a story that is? an?"inconceivable mystery"? -- that is, it's unbelievable????It?requires the suspension of normal?logic and of the?scientific rules that govern?the world as we know it.

Scripts about parallel universes (or time travel)?can be very?hard to write in a way that will make them believable.?? And it can be even harder to write a logline?in 30 words or less that accomplishes the same feat.

I presume the "strange world" to be the parallel universe.?There seems to be a? cause-and-effect link between the severed hand and the headless body that ?leads to the discovery of a parallel world.? But I have no idea what it is.? How does one lead to the other?