In this homage to classical music, Vincent stumbles across a former resident of "The World Below", once a gifted pianist, but now a drug addict struggling to survive in the city. Through a series of flashbacks it is revealed that the young man feels responsible for the death of one of the helpers, his piano teacher. In the end, he refuses to accept the love and forgiveness waiting Below, though Vincent assures him no one blames him for her death.
Catherine longs for Vincent to escort her to a secret hidden place that she had as a child in Connecticut. Yet Vincent realizes that there is too much danger if he were to venture above. Vincent then begins having visions of an angel who takes him on a journey of what life would have been like if he had never existed with one of them having involved Paracelsus successfully taking over "The World Below."
Vincent rescues a man named Dimitri from drowning and brings him to "The World Below" to recuperate. Little does anyone know, Dimitri is carrying the pneumonic plague which soon becomes an epidemic amongst the residents of the tunnels and begins taking lives. Meanwhile, Catherine discovers that Dr. Peter Alcroft is an old friend of Father when it comes to helping the infected.
When "The World Below" is planning an annual winter event called Winterfest with Catherine and the other "Helpers," Paracelsus plans to crash the party. Catherine and Vincent suspect that someone at the party is actually Paracelsus in disguise and must find out who it is.
After meeting with a pregnant former prostitute named Lena who called a help line, Catherine convinces Vincent and Father to welcome her to the "World Below" where she can have her baby and be part of the community. Before actually meeting Vincent, Lena hears stories about him and once she encounters him in person, a crush develops into something more for her. After the baby is born, she offers her love to Vincent, who rejects her advances gently but firmly. She runs away, leaving her child behind. Catherine tracks her down and convinces her to return to her new home, where in a naming ceremony Lena calls the baby girl after Catherine.
Catherine learns that the deaf girl Laura, a former resident of "The World Below," has fallen in with a gang of angry young deaf people. The leader of the gang "Lincoln" is being investigated for his involvement in the murder of a security guard. Laura's boyfriend Jerry is actually an undercover police officer who has infiltrated the gang. He reveals to Laura that he is a cop and not really deaf but that he wants to be with her. She runs off and when she returns, she discovers the gang has found out who Jerry is and they have sentenced him to the punishment of "sticks and stones." He is rescued by Catherine and the police who are searching for him, as well as Vincent who is summoned by one of the helpers.
Michael, a young man who has lived in "The World Below" since he was a child, is accepted to college in New York. Catherine invites him to stay with her but the relationship becomes complicated when he falls in love with her and reveals the truth about his parentage.
A young man gets lost in the tunnels and is found by the inhabitants of "The World Below." They are unsure if they should let the boy go in the event that he doesn't keep their existence a secret.
Vincent's "brother" Devin Wells returns with Charles, a deformed man with Proteus syndrome (Elephant Man disease) who seeks sanctuary. He was abused by his brother Eddie and Devin tells Catherine that Charles was once imprisoned in the circus under the billing of the "Dragon Man." Charles is unpredictable and inadvertently injures several people before Devin decides to take him back above and look after him himself upon looking for someplace where nobody can find Charles.
A resident of "The World Below" named Kanin heads to a store in "The World Above" and is recognized by a woman as the man who killed her son years ago. Although Catherine doesn't want to force Kanin leave his family, Vincent thinks he should take responsibility for his crimes and give closure to the woman who lost her son.
When a group of homeless people find their way into "The World Below," Father decides to welcome them. Vincent is wary about this as it seems that the homeless people are violent and disturbed.
When Catherine's father dies of a stroke, she retreats to "The World Below" to grieve. She asks Vincent to let her stay with him and not return to "The World Above" and though he agrees, he knows that eventually she will need to go back. One night she is visited by her father's spirit in a dream and she comes to terms with losing him, which gives her the strength to return to her world.
A former resident of "The World Below" named Lisa, now a world-famous ballerina seeks refuge in the tunnels to avoid testifying against her boyfriend. The situation is complicated by memories from when she and Vincent were teenagers. She would dance for Vincent and he fell in love with Lisa but one night while she danced, Vincent hurt her when they embraced and he wouldn't let go. This memory is for Vincent a chief reason why he fears a physical relationship with Catherine. Catherine is able to console him and to convince Lisa to go back above to face the authorities and do what she can to help put away the man she's involved with for his crimes.
While in a bookstore Catherine encounters an eccentric young artist named Christopher who she later finds out is "dead." He keeps popping up in her life, even once at the tunnel in Central Park, where he sees Vincent. Though he seems harmless, Catherine is convinced he has faked his own death to reap the benefits of being a "dead artist." His friend, the bookshop owner, swears he is dead, having identified the body himself and claiming she's not the first to have seen his ghost. She tracks down Christopher's paintings to a warehouse where Vincent joins her and Christopher seems to appear out of thin air just long enough to show them where to find his work and then mysteriously disappears. Catherine arranges a show at a gallery, which is a great success. The gallery owner presents her with a wrapped painting that has her name on it. It turns out to be an oil painting Christopher did of her and Vincent. Vincent comments that the paint is completely dry, something that can take years for oil paints, adding to the air of mystery.
Catherine is pursued by a stalker who can see her apartment through a telescope. On the night of their second anniversary, Catherine and Vincent are on the balcony when she receives a phone call from her friend warning her she is in danger. A moment later, the stalker calls, claiming he can see Vincent. Catherine tries to find out who he is and though she reveals to Joe that someone is watching her, she refuses to involve the police, in order to protect Vincent. The stalker eventually grabs Catherine, despite her, Joe and Vincent's efforts to keep her safe and leaves her unconscious body in the trunk of his car in a lake. Catherine is at the edge of death when Vincent arrives, dispatches the stalker and rescues Catherine from her watery grave.
Catherine must travel to California as part of an investigation for the DA's office in New York. She and Vincent discover just how strong their bond is when Vincent is able to warn her telepathically of an impending attack while she is trying to bring a witness back to New York with her.
Catherine and Joe have been working on a case involving a high power Wall Street celebrity whose child abuse turned deadly resulted in the death of his son. Catherine is named the prosecutor in the case and she soon must deal with the challenge of trying to get the child's mother to testify against her husband
Elliot Burch has returned to New York and finds that his life his in danger. It seems a death squad has followed him to New York due to dirty dealings that he was involved in Central America. He soon gets Catherine involved who puts her own life on the line to help him.
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 on his own to put an end to their madness.
Catherine is attacked by two men in the park who are disguised as police officers. Vincent comes to her rescue only for it to be a set-up caused by a shadowy figure who has hired an investigative reporter named Bernie Spirko to uncover the existence of Vincent.
Following the death of Bernie Spirko in the last episode, 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 in the final confrontation with Paracelsus.
Following the death of Paracelsus, Vincent comes to terms that the beast within him is causing him to lose control.