Kenzi heads to an underground party - that's underground, "secret," and underground "subterranean" - in order to investigate the disappearance of several partygoers right and left. She and her longtime bud, Ozzie, take in the occult show that summons Bloody Mary, The Slender Man, and the Sewer Alligator. The lights black out, and kids start disappearing all around them - including Ozzie. Kenzi manages to escape, and Bo interrupts her romantic tryst with her new girlfriend, Lauren, to help find the missing teens.
They're not the only ones on the case: Dyson gets introduced to his new partner, Tamsin, when she shows up to tell him about the very same disappearances, and one body carrying the mark of a sewer alligator. Just one hitch in the working relationship between the new partners: Tamsin is Dark Fae. At the crime scene, Dyson picks up Bo's scent, and the two call her in to the station, but while there, Tamsin ambushes Bo with pictures of the man she killed during a blackout - something Dyson has not clue about - but since the kill was Dark Fae, Tamsin takes it very personally. Bo says she knows nothing about it, and tries to tell them about the alligator Fae they met underground, along with scores of Fae infected by some mysterious disease. The alligator, Atticus, said he and the others were infected after being quarantined, then gassed our heroes and delivered them, unconscious, to their own home.
Tamsin isn't buying it, but Bo sneaks out of the station and heads home to get Kenzi when who should stumble in but Ozzie, Kenzi's missing friend. He has no recollection of being abducted, and when his nose starts bleeding, Bo knows she has to take him to the Dal where Trick has a Weaver - a mystic who can plumb the unconscious - to find out the truth. Ozzie was abducted and taken to a shipyard, so Bo and Kenzi race to the location seen in the Weaver's vision, and discover dozens of young people thought missing. They bring them in to the station, where Tamsin offers a contrite apology, but Dyson's chief redoubles his order to exterminate those underground.
This directive sounds off to Dyson, who consults with Hale and learns that the infected Fae were only meant to be relocated. This news sends him racing after Kenzi and Bo, who he knows went back to the sewers to set things right. He arrives just as his chief, Chief Hamlin, puts the women in his thrall. Hamlin is actually a Fae who enslaves people in order to feed off them - sometimes known as the Slender Man, best known as the Pied Piper. When Atticus hears his voice, he arrives to attack Hamlin in revenge for the death of his wife and child. Atticus has gone blind from so many years underground, and this makes him impervious to Hamlin's powers. He sinks his teeth into the man's jugular as Bo and her friends scurry to safety. As soon as Hamlin is dead, all those under his thrall are released from the spell. Hale relocates the newly-restored Fae, and all is well.
...almost all. Bo lies down for a session with the Weaver in order to find out what's been giving her night terrors, and the Weaver sees something too horrible to repeat. Bo wonders if it might have some connection to the accusation Tamsin made about the messy feed, but it seems she'll have to wait for any real answers, as the Weaver hurries away as if her life were in danger.