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.
Catherine reaches Vincent in the catacomb and rescues Vincent from his madness. However, as Vincent begins to recover, he realizes he has lost his bond with Catherine and is unable to sense when she is in danger. Meanwhile, a deadly explosion lands Joe in the hospital where Catherine discovers that she is pregnant and debates how to tell Vincent, but as Catherine begins investigating the explosion it leads to her being kidnapped by a powerful crime syndicate led by a shadowy figure.
Following Catherine's death on the rooftop, Vincent takes her back to her apartment and leaves her on her bed. Later, as Vincent is grieving, Joe begins an investigation into Catherine's murder, hiring a private detective Diana Bennett, who encounters Elliot Burch and also members of the Tunnel World at Catherine's funeral. Meanwhile, Vincent's past demons re-surface as well as memories of Catherine who soon begins to search for his son.
Vincent has begun his quest to find his son and makes his first move by revealing himself to Elliot. Meanwhile, Diana has become immersed into Catherine's life and soon discovers the entrance into the Tunnel World in the basement of Catherine's apartment building and Elliot discovers the role Moreno played in Catherine's death which leads to a confrontation resulting in Vincent being shot.
The man who killed Catherine and took her baby sends an assassin to get Vincent. He manages to find the underground world and goes looking for him. Vincent upon learning of his presence tries to lead him away from the others.