The first season of “Mindhunter” captured my imagination, dark though its story material is. Serial killers, the tracking down of them, that’s what the series relates, through the eyes of two FBI psych researchers (plus an engaging second string of FBI-ers). The second season’s opening episode hit hard but then the next four episodes seemed to “lose direction.” What then of the final four episodes running down the main underlying story of a brazen, clever serial killer of black children in Atlanta? Thankfully the series’ brooding investigative edge returns, all the acting remains superior (with the two main actors, Jonathan Groff and Holt McCallany, outstanding), and the dark cinematography is spellbinding. The climax, vindicating though it is, partially slaps down the new FBI profiling unit and the closing scenes are bleak indeed. Season 3 beckons.