Based on the book by FBI agent John E. Douglas — the character Holden Ford is based on him — S1 set up a format of serial killer interviews interspersed with a little too much personal drama in Holden's love life. Their portrayal of Ed Kemper, the Co-Ed Killer, was especially impressive. Sympathetic sociopath.
S2 starts out more of the same, interviews with Ed Kemper again, Charlie Manson, David Berkowitz, and several others, building up the theory of sexual compulsion; interrupted by Wendy trying to have a relationship, Bill's kid becoming a budding sociopath and his wife's high-strung nonsense so Bill has to fuck off of work every weekend to try to be a dad. All the personal stuff's a waste of screen time.
But then it starts to focus on the Atlanta Child Murders, brings back agent Jim Barney for local expertise. The long, half-assed struggle as they don't really know how to profile yet, or have adequate support from Atlanta PD (the photocopy flyer story is hilarious/awful; Kinko's was around and used by every indie zine and punk band, and by the alleged killer, and by BTK in the before-credits scenes, but the cops and FBI can't get anything done in less than geological time). There's a ridiculously long stakeout that finally gets them their one suspect, and the difficult politics in getting him prosecuted.
In reality, Douglas got censured for saying the suspect "fit the profile … looked pretty good for a good percentage of the killings". Holden doesn't get to shove his foot that far in his mouth. And we still don't know how many people the suspect actually killed; there's better evidence of some, but he definitely didn't kill them all. Someone else killed at least a dozen kids and got away with it.
★★★★☆ — needs focus on the criminal profiling, not the quasi-fictional characters on the side. Tends to be too gentle on Holden Ford/John Douglas who is seriously quite a flaming asshole.
There's also a podcast, Atlanta Monster about the case, S2 is going into Zodiac.