The television police procedural is a genre, and like any genre, it makes an implicit contract with its audience.
Chiefly, that contract is about plot. Here's what you'll get, it says. Each episode, a crime will be committed, investigated with a certain amount of technical detail, and ultimately solved. That's it. We may introduce some embellishments — a chewy performance here, an out-of-left-field twist there, or maybe a tiny amount of character development — but week in and week out, we'll stick to the parameters.