(Some spoilers ahead for season 2 of Netflix’s “Mindhunter”)

The first season of “Mindhunter” on Netflix included a bunch of strange scenes about a guy who didn’t seem to have anything to do with the main plot. And that’s true — these scenes were only thematically linked to the main story, because that guy was Dennis Rader, aka the BTK Killer.

Season 2 changed the game a bit, however. In the second episode, Bill Tench (Holt McCallany) of the FBI’s Behavioral Science Unit flies to Kansas to look into the BTK case — which has hit a lull as the serial killer has been dormant for a while — and in the rest of the season they call back to what they know about BTK as they investigate the Atlanta Child Murders.

The highlight of that Kansas visit was Bill’s early morning chat with the only known survivor of a BTK attack, Kevin Bright — whose sister was killed in the same attack. And then Bill heads home to Virginia without making any real progress beyond collecting info for the BSU’s profiles of serial killers. They talk about BTK occasionally through the rest of the season, but Bill never resumes that investigation.

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The vignettes continue, however. Over the course of season 2 of “Mindhunter” we are shown that Rader tried to walk on the straight and narrow because his wife caught him engaging in auto-erotic asphyxiation while wearing some very…interesting clothing. By the end of the season we see his attempts to reform and turn himself into just another regular guy fail, and he gets back into his odd sexual proclivities.

This whole thread is interesting because we know that BTK isn’t going to be caught for more than two decades after the events we’re seeing — Rader was finally arrested in 2005. So unless they do a major time skip at some point that’s not an event we’re going to see on “Mindhunter.” We’re still in 1981 at the end of season 2, after all.

So what’s the purpose of this thread? McCallany explained it a bit in an interview with Esquire.

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“One of the things that I think separates our show from other shows is that we show the reality of police work,” McCallany points out, “and the reality is that they don’t always get the guy! They didn’t catch the Zodiac. They didn’t catch Rader until 2005, and only because he made a really stupid mistake, sending a floppy disk that was traceable, directly to the police department in a kind of Son of Sam style quest for notoriety.”

Rader had ended his murder career in 1991 but began correspondence with the press and police again in 2004, taking credit for a murder that had not been linked conclusively with BTK. It was one of those correspondences, on an old 3.5-inch floppy disk, that eventually did him in when police traced it back to him.

With it seeming unlikely that the story “Mindhunter” tells would extend all the way to 2005, the recurring focus on Rader becomes more of a thematic thing than a plot thing for the show. Here we have a killer living his life and in some important ways defying the profile that Holden Ford (Jonathan Groff) and co. have constructed for serial murderers. The BSU is not perfect, and they get some very big things wrong.

For example, Holden says a number of times during season 2 that serial killers are incapable of living normal lives, but Rader directly contradicts that idea — he’s got a job and a family and holds a leadership position at his church and all that. In Ed Kemper’s only scene in season 2 he points out something very important to Holden — that all the BSU’s insights come from serial killers who got caught.

It’s an important lesson for Holden, who is always convinced he is right about everything. But Holden doesn’t actually learn that lesson — during their investigation into the Atlanta Child Murders for the rest of the season, he and Bill constantly argue about the merits of developing leads and eliminating them vs just going all in on the profile Holden established early.

In the end, “Mindhunter” casts plenty of uncertainty over Holden’s conclusions about that case. And the BTK vignettes drive that point home. Holden and the BSU are not infallible, and there is so much they still don’t know about serial killers. And making that point is at least partially, in my opinion, why the BTK vignettes persist.