In Detroit, the three nights surrounding Halloween are collectively known as Devil's Night. Vandals take over the city, smashing windows and lighting fires. For the past three years, UnSub Kaman has used the mayhem as a cover for his abductions and murders. Our team only has the three nights of Devil's Night to catch Kaman before he disappears again for another year.
On the first night of Devil's Night, Kaman takes victim Tony Torrell into an empty warehouse in Detroit’s Rivertown district. There, he ties him up and burns him alive. While Morgan wants to start the investigation off by looking at firefighters and first responders (the most likely occupation for a firebug), his investigation if complicated by the fact that a thousand-strong civilian organization called Detroit's Finest has been patrolling the area.
They decide to take the investigation in a new direction by talking to Tony Torell's wife, Kierston. Rossi gets her to recall the night of the abduction, and they are able to learn that the UnSub's body is probably badly burned. He only commits crimes on Devil's Night because his disfigurement seems like a Halloween costume, so he can blend in.
As the police scour the city, Kaman abducts Chris Edwards, his second victim. But when he runs into a police blockade on the way to the abandoned part of Riverton, he panics and has to hastily burn him in a back alley. Meanwhile, Garcia has uncovered Kaman's identity and the race is on to catch the now-devolving UnSub.
The team tracks Kaman to a restaurant called Jay-Mo's, which is owned by the father of Tracy, Kaman's girlfriend from before the accident that lead to his disfigurement. As Kaman tortures Jay-Mo, we learn that much of his reign of terror has been due to feeling abandoned by her. Finding her is his endgame now. By the time the team makes it, Hotch is barely able to save a near-dead Jay Mo from the restaurant before it burns to the ground.
Much closer on Kaman's trail now, the police surround Tracy's house while Kaman is inside for his final confrontation. Since the house has been doused in gasoline, though, the police cannot fire a single shot without risking the lives of Tracy and her family. It is Hotch, via megaphone, who ends it by getting Tracy to admit to Kaman that they had a son together (who he's never met until now), upon whom Tracy bestowed the middle name of Kaman.
The episode is bookended with Hotch and Jack getting ready for Halloween. In the end, Jack decides to dress up like his father – a real hero.