A disgraced ex-cop, offered the chance to regain custody of his son must partner with a soon to retire ex-prosecutor to catch a serial killer before he kills more children, digging for the truth they unearth a hidden layer of personal and professional secrets of those involved from within and outside the law.

I like this logline a lot but seems it has several plots and not sure which one is the main one. Let’s try simplifying it.

A disgraced ex-cop [CHARACTTER], offered the chance to regain custody of his son must [ACTION/GOAL] partner with a soon to retire ex-prosecutor [CHARACTTER] to catch a serial killer before he kills more children [ACTION/GOAL], digging for the truth they unearth a hidden layer of personal and professional secrets of those involved from within and outside the law [ACTION/GOAL].

Maybe something like

“A strait-laced prosecutor has to partner up with the disgraced ex-cop who he got fired in order to save his kidnapped son from a serial killer.”