The team is called to LA to investigate a series of home invasions turned homicides and they posit that the killings are all being done by the same Unsub. At the same time, because of the heat wave going on, there's a series of rolling blackouts going through the city. The team discovers that the Unsub is taking advantage of the city's rolling blackouts to pursue his victims; he has his pick because everyone is vulnerable. The team realizes that in order to stop the Unsub from killing when the lights go out they need to stop the rolling blackouts. The risk they take in doing this is the potential for an ENTIRE city blackout which is what ultimately will happens by Act Four.
The team figures out that the Unsub chooses to kill in dark for a reason. They look at previous home invasions where the power was cut and Garcia finds that this MO was popular 25 years ago where there was a series of home invasions/homicides in San Diego, Orange County and LA. No one put it together before because they never realized that the power being cut was actually the characteristic that linked all of them together. What the team also discovers is that this Unsub has been active since then but he's being working his way throughout California killing people at random.
So why is he back in Los Angeles now? There's something personal for him to come back to LA. The lead detective on the case, Matt Spicer, is a hot shot, young mid-30s cop. He's basically touted as the hero of the city, consistently praised for being such a great detective at such a young age. He happens to be a survivor of one of the previous home invasions, 25 years ago, where the Unsub killed his single mother and left him as the sole survivor. This was the reason he became a cop – for vengeance and to catch serial killers like the one that killed his mother. The Unsub is now back because he loves taunting his victims, screwing with them. He's seen Spicer on the television. He is the head of the homicide unit and the Unsub knows that the only reason he's there is because of what the Unsub did to him. He returns to LA just to screw with him. And the ultimate reward for the Unsub is that this detective now has a daughter of his own, Ellie.
By the end of the episode, the detective is back at his old house, the original scene of the crime, with his daughter, Ellie, his sister Kristin and Morgan. The Unsub is there as well and forces Spicer to beg for his life or else he'll kill his daughter. Morgan is restrained and unable to do anything to help. Spicer begs for his life and the Unsub shoots him in the head and takes off with Ellie. And this is where we leave the season only to pick up next year with our team chasing the Unsub and the girl.