Portland is in the grips of a brutal crime wave and petty thefts and assaults are at an all-time high. On the outskirts of town, two thugs rob and brutally beat a gas station attendant to the brink of death. As the news story consumes the residents of Portland, the gas station attendant's mother prays to the heavens that someone will seek revenge on her son's attackers. Across town, an unseen Wesen creature hears her cries and springs into action.
Meanwhile, Juliette is frantically trying to determine who sent the mysterious email to Nick. Who is "M?" When Nick returns home from the station, Juliette questions him about the cryptic email. Nick explains to Juliette that "M" is his mother. Juliette is shocked to learn that Nick's mother is still alive, but at least she knows that she can still trust Nick. Nick has bigger issues to deal with though; according to the email, his mother is in trouble. Juliette manages to track the source of Nick's mother's email to Slovenia.
The following night, the two gas station thugs hold up a liquor store, once again beating the store clerk nearly to death. This time, the crime does not go unpunished. Just around the corner from the liquor store, the two thugs are confronted by a shadowy Wesen, who effortlessly tears them to shreds. Hank and Nick arrive at the scene to survey the gruesome damage. They immediately suspect that a Wesen is to blame, but who? And what type of Wesen would create this much carnage?
Nick and Hank pay a visit to Ray Bolton, a rare Höllentier Wesen who is known in the neighborhood for his frightening dogs and brash behavior. Ray has a history of violent crime, so he may be a key suspect in the murder case. Ray immediately puts up a fight when the detectives try to confiscate his dogs for testing, so he's arrested as well.
Ray would be the prime suspect in the murder investigation, except for one catch: while Ray is in custody, the vigilante Wesen strikes again. This time, the beast brutally murders a thug who attempts to assault a woman on her way home. This rules out Ray as their suspect. At the scene of the crime, a witness blames the attack on El Cucuy, a legendary Hispanic beast who seeks vengeance on evildoers.
Back at home, Juliette tells Nick more about El Cucuy, the mythical boogeyman from Mexican folklore. Hoping for more information on El Cucuy, Juliette and Nick visit Pilar, the woman who helped with the La Llorona investigation once before. Pilar tells them that El Cucuy responds to cries for help and seeks vengeance against criminals and tyrants. Nick knows he's on the right track, but he'll need to figure out who El Cucuy is.
Nick heads to the Trailer to learn more about the El Cucuy Wesen, but his search comes up short. Juliette suggests that maybe El Cucuy isn't actually a Wesen, but something else altogether. Back at the station, he watches security footage from a nearby bus and notices something peculiar: an old Hispanic woman was at both scenes shortly before the brutal murders. Nick and Hank bring Mrs. Garcia to the station for questioning.
After a quick stop at Ray Bolton's house, the woman disappears from the car. Moments later, El Cucuy attacks and murders Ray inside the house. Nick bursts in to find Mrs. Garcia covered in blood just as she un-morphs from her El Cucuy form. Back at the station, Mrs. Garcia tells the detectives that nobody will believe she's responsible for the gruesome murders, and she's right. Nick and Hank are forced to release her, allowing El Cucuy to prowl the streets again.
Nick and Hank battle a powerful, vengeful Wesen who is determined to clean up the mean streets of Portland by murdering street thugs.