When the tunnel world is having an annual event, Paracelcus makes plans to crash the event and do something. When all of the participants arrive, including some friends from the surface, things seem to be OK until Narcisa shows up. It seems she had encountered Paracelcus earlier who had in his possession someone's head who then tried to kill her. They assume Paracelcus is there in disguise. Catherine and Vincent deduce it must be someone from the surface but there are only three whom he knew before he was banished so they try to figure out who could it be.
A young man gets lost in the tunnel world and he is found by the inhabitants. They're uncertain if they should let him go because they're not sure if he can be trusted to keep their secret. But when a search party comes looking for him, they decide to try to convince him to keep their world a secret.
Kanin, a resident of the Underworld, goes into a store on the surface and is recognized by a woman as the man who killed her son, years ago. She then goes to the police and D.A. demanding that they bring him in. Catherine speaks to them and Kanin doesn't want to leave his family. But Vincent tells him that he should take responsibility for his actions.
Two rich kids have been targeting and murdering prostitutes for fun. Late one night, Vincent witnesses their latest murder and is unable to stop them. Catherine soon puts together a case against them but has a hard time putting them away. Meanwhile, Vincent has set out on his own to put an end to their madness.
Catherine is attacked in the park by two men disguised as policemen. Naturally, Vincent comes to her rescue but it soon becomes apparent that this event was a setup to get Vincent out in the open and has all been orchestrated by a shadowy figure who has hired an investigative reporter to uncover the existence of Vincent.
Following the death of investigative reporter, Bernie Spirko, Elliot Burch and Catherine set out to find the man who hired and killed Spirko. Little do either of them know that the man they are seeking is Paracelsus and he has set into motion the next stage of his diabolical plan. He lies to Vincent about his birth which brings Vincent to the verge of madness.
Following the ordeal with Paracelcus, Vincent moves forward toward a stress-aggravated mental collapse. Vincent continues to come to terms with the beast in himself and finds himself losing control of his thoughts. He finally decides to go into seclusion into a deep chamber far back through the catacombs where madness takes over his mind. Meanwhile, Catherine, Father and the rest of tunnel dwellers try to find some way to help Vincent.