Detective David Creegan returns to work after recovering from a gunshot wound to the head. Assigned a tough female partner, he is given the job of tracking down a child abductor responsible for the disappearance of three boys.
The team hone in on a suspect who is linked to a string of similar killings. However getting hold of enough evidence to tie him to the crimes remains elusive. Meanwhile, time is running out for the kidnapped boys.
A woman is found burned to death with strange markings on her body, and suspicion falls on a man obsessed with fire - but subsequent events seem to rule him out of the investigation.
Creegan and Branca are forced to enlist a murderer to help them solve the deaths of three women found beaten to death outside strip clubs.
The team searches for a serial killer whose gruesome crimes have earned him the appropriate nickname of the Hangman. Branca rekindles her romance with Stephen Laney, a reporter covering the case.
Creegan tries to resolve his family difficulties, and the team tracks down a disgruntled former soldier preying on Iraq war veterans who served alongside him in the same army unit.
Creegan and Branca investigate a case of horse mutilation and track down a quartet of youngsters addicted to a type of veterinary medication known as K.
A serial killer is released on parole and wastes no time in striking again, leaving the latest victim's father grief-stricken and vengeful, which prompts Creegan to consider the safety of his own family.
Branca uses her linguistic skills to infiltrate a Ukrainian baby-smuggling ring, while the rest of the team tries to solve a string of murders committed by a shadowy contract killer known only as the Morning Person.
The OSC unit heads for Creegan's native Boston to help local officers find a serial killer who seems to be selecting his victims on the basis of their religious faith. Father Bob Di Fabian claims to have been nearly killed by the man while taking his confession, but Branca does not believe the priest because of her past experiences with Catholicism.
Creegan's friend Cyril is arrested on suspicion of being involved in a spate of fatal knife attacks on smartly dressed men, but a lack of evidence hampers the investigation - until an eyewitness account places him at the scene of one stabbing.
Creegan makes a decision about his family. Rivers suffers a personal loss, and the team visits Washington to search for a killer who cuts out women's hearts and leaves them for their husbands or boyfriends to find.
The San Francisco team investigates the mysterious deaths of patients in intensive care, who were being treated by a doctor known to be sympathetic to mercy killing.
