Street savvy Assistant DA Catherine Chandler hails from a rich family and works for her father’s law firm. After leaving a party early, she’s mistaken for someone else and is pulled into a van full of thugs. They slash her face and beat her up, then push her out of the van, leaving her for dead. A shadowy figure appears and carries her to secret set of forgotten tunnels below the city.

A subway slasher that kills with claws and leaves the victims looking as if they were mauled by a bear is on the prowl. When Catherine learns about the case, she starts to suspect Vincent, as she remembers how he rescued her from the attackers in the safe house.

Vincent helps the residents of an apartment building defend themselves from thugs trying to chase them out so the building can be torn down—unaware the mastermind is Elliot Burch, a megabucks developer courting Catherine.

Upon being injured in an explosion, a half-blind Vincent is captured by a vicious street gang. Catherine, with the help of her self-defense teacher Isaac, set out to find Vincent.

Able to blend in among 'The World Above' on Halloween, Vincent becomes the guardian of an Irish peace activist targeted for assassination.

Catherine has a nearly fatal confrontation with an ex-tunnel dweller, named Mitch Denton.

After Vincent is accidentally caught on film. he is captured by two men. One, Dr. Edward Hughes, is a scientist desperate to learn the truth about Vincent. The other, grad student Jonathan Gould, believes Vincent is a freak of nature, and wants to reveal him to the public and become famous. Meanwhile, Catherine considers a new job in Providence.

When Father ventures to 'The World Above' for the first time in decades, he is arrested for murder. Catherine and Vincent must dive into Father's past—where they learn of his true identity, Jacob Wells, a physician; his blacklisting during the McCarthy era; and, of his long-lost wife—to clear his name and reunite him with his now-dying wife.

While investigating a businessman's mysterious death, Catherine becomes a target for a mysterious voodoo cult, who makes her see and experience terrifying hallucinations. They then turn her against Vincent, making her believe he is responsible for the visions.

Catherine learns that renowned orphanage Ridley Foster Home holds a dark secret: selling orphaned children to criminals who force them to steal. She saves one such orphan, Eric, and brings him to 'The World Below' where he's welcomed by Father—who promises his missing sister Ellie a similar refuge—while Vincent helps Catherine find Ellie and end the child-selling racket.

Upon witnessing a shooting, deaf Laura Williams must decide if she should leave the sanctuary of 'The World Below' to free an innocent man. While Catherine is investigating the same case, she is attacked in her apartment by the men behind the murder.

'The World Below' shun Mouse for his acts of thievery, until they need him and his mechanical and engineering intelligence to help rescue Father and Vincent following a cave-in. Mouse, in turn, needs Catherine's help, which sends her reluctantly to Burch for specialized construction equipment that may free Father and Vincent in time.

Chinatown is the setting of a 'Romeo and Juliet'-like story of a woman who is friends with Vincent and who has fallen in love with one man, but—through an arranged marriage—has become engaged to the son of a Triad leader.

Catherine believes a deadly new hallucinatory drug on the street is supplied by someone from 'The World Below'. Catherine, Vincent and Father discover Father's former friend Paracelsus—who originally helped him build 'The World Below'—is behind it.

Joe spearheads the investigation of a major firm, so the firm's attorney begins using dirty tactics to derail Joe, and Catherine discovers that Joe's new girlfriend is a vital to the conspiracy. Meanwhile, Vincent travels to a deep chamber to find a gift for Catherine, in celebration of the one-year anniversary of their first meeting.

New Assistant DA Devin Wells turns out to be Vincent's long-lost childhood friend from 'The World Below'. His vagabond way of life began after a falling-out with Father over his endangering Vincent aboard a park carousel. The uneasy reunion leads to the revelation of one of Father's long-kept secrets, but ends in their reconciliation and with Devin now knowing he has a home to return to.

Vincent has foreboding dreams when Catherine's ex-fiancé Steven Bass returns, and with good reason: Catherine is barely aware Steven—whose parents are now dead and who's dying of brain disease, but whose becalmed manner at first belies that he's still a control freak—is obsessed with making her the wife she doesn't want to be, and goes to troubling extremes to make it happen.

A necklace Mouse gifts Catherine turns out to be made of antique gold, found in a buried ship Mouse discovered by accident deep in 'The World Below'. When he shows Winslow and stone-worker Cullen, it leads to an unheard-of spell of raw greed among their fellow tunnel dwellers, especially when Cullen tries to claim the ship's treasure as his own for sale, until Catherine, Vincent and Father find a surprising resolution.

Vincent and Catherine become involved with a gypsy boy banished from his tribe because of a crime his father allegedly committed and who claims that his father is actually innocent.

Catherine receives an unexpected visitor on her balcony: Paracelsus's minion Erlik, who captures her in a plot to lure Vincent to the lower parts of 'The World Below' and kill him so that Paracelsus can overthrow Father and take over the tunnels. Despite knowing what Paracelsus would do, Vincent goes to rescue her with the help of Pascal, Winslow, and Jamie.

Burch's great construction project "Burch Tower" threatens the existence of 'The World Below', and Catherine agrees to marry him if he will halt construction.

Depressed on the anniversary of her mother's death, Catherine struggles emotionally between her relationship with Vincent and her working life, enough so that Vincent tries to end their relationship.