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Jim, Blair and Cassie are called out to an apparent professional vigilante killing of a union official, Bill O'Doyle, who had recently escaped prosecution on charges of killing a rival union boss. The body was beaten, strangled and then shot in the heart with a .45 caliber handgun, an MO which brings back disturbing memories for Cassie of Warren Chapel, "The Avenging Angel", a serial killer who had administered punishment on three criminals in San Francisco who had escaped the courts on a technicality.

With great trepidation Cassie visits Chapel at the Connover detention facility where his psychiatrist Dr. Burke confirms that Chapel is kept segregated, except for his weekly group therapy sessions, and has no visitors or phone access. Chapel nevertheless seems to have knowledge of details of the killing never released to the press.

In Simon's office, Cassie reveals what she knows, and Simon tries to reassure her that Chapel is in maximum security and it just playing with her. Nevertheless, Jim and Blair feel it is worthwhile pursuing and have come up with a likely target for the killer, Joseph Bellini, who was accused of white slavery and murder, but walked on a technicality. Simon agrees to setting up a surveillance team on Bellini, but just before 9pm that night, Simon calls to say that Bellini's lawyer has issued a Cease and Desist order, and the stakeout is officially terminated. However, as Jim and Blair are leaving, they find Cassie's van parked outside Bellini's place and when Jim hears a scuffle from inside, they find Bellini dead on his living room floor.

Cassie return's to question her old nemesis, but this time Jim and Blair are watching on closed circuit TV. When Cassie fails to get him to reveal any more clues, Jim and Blair question the warden about possible access to someone on the outside who could be committing the murders. They decide they need to put someone in undercover and Blair volunteers, although Simon in difficult to convince. Blair insists that he is familiar with the facility and the therapeutic practices as he spent a semester working nights at Connover researching a paper. Simon is adamant that they need to find a police officer for the job but Cassie insists that Chapel will smell a cop and Jim thinks it is their best bet. Reluctantly, Simon agrees and Blair goes undercover as a patient in the hopes that Chapel will let his guard down.

In the meantime Jim's background check on Chapel with the SFPD reveals that Chapel actually tried to kill Cassie and shot her in the shoulder right before he was arrested. He insists on knowing more, as Blair's life is in danger. Cassie reveals that since Chapel blamed her for his arrest, it was one of the reasons why she moved to Cascade to start afresh. Back at Connover, Chapel is escorted to a deserted laundry room by a guard called Harris who has been helping him in return for a hefty payoff. When Harris leaves, Chapel pulls a concealed telephone handset from a ceiling panel and hooks it up to the phone lines where he calls his female accomplice and insists on vicarious details on the latest killing.

Blair is is introduced to the inmates at the next therapy session and goes into his act as an obsessive compulsive who responds to the directives of his "higher power", yet Blair also feels the impact of Chapel's freakish personality. In the meantime, Jim has started to conduct a thorough background check on the staff at Connover. Despite Jim's reservations, Simon insists that Jim secure Cassie's assistance. On the scene of the next murder, Cassie tells Jim that she is experiencing nightmares and begs him to pull Blair out but she is unwilling to get further involved, so Jim heads for Connover.

Meanwhile, Blair overhears another conversation between Chapel and the killer talking about making their escape but when he tries to call it in Chapel discovers him. Blair hits Chapel with the telephone receiver and flees through the hospital corridors, eventually running into Jim coming in the opposite direction.

Back at the loft, Blair is insistent that he still wants to help and takes over looking through the case files. When Cassie arrives, she recognizes the face on one of the files, the ex-army sister of the gas station owner who was killed by Chapel's first victim. Cassie remembers that the sister seemed fixated on Chapel and attended every day of his trial. Jim further uncovers that the suspect, Jean Carpenter, was discharged from the army for psychological imbalance. They plan a raid on her address.

Back at Connover, Harris panics when the police start to question the staff and he realizes what Chapel might be involved in, but Chapel threatens Harris into helping him plan his escape. When Dr Burke realizes that Chapel has escaped he tries to call to warn the police that Chapel has probably joined Carpenter, just as the police are raiding her house. At that moment the SWAT teams are assaulted by a series of landmines and as a the suspects try to escape the scene, the police shoot at the vehicle, which explodes on impact, killing both occupants.

Apparently the case is closed when the bodies are revealed to be a man and a women. Jim agrees to take Blair and Cassie out for a meal and they wait at the loft while Jim and Simon wrap up the loose ends of the case. They send out the dental records and some metal fragments found at the scene for analysis, but when Jim goes down to check the evidence bags he remembers Blair mentioning the metal pins in the leg of one of the case files he was looking into -- Harris, the guard at Connover. A call to Dr Burke confirms that not only did Harris have a titanium pin in his leg, but he also didn't show up for his shift.

While Jim races home, back at the loft, the phones and power is cut and as Blair and Cassie attempt to escape, Chapel confronts them at gunpoint. Chapel accuses them of killing Carpenter, whom he says was an extension of himself. Chapel is waiting for Jim, who Blair assures him is coming.