Each episode begins with a flashback scene informing viewers of the featured year it takes place. A set of characters are revealed in a seemingly mundane situation. The viewer is then shown the corpse of the victim as he/she was found. It is always one of the people introduced in the previous seemingly-innocent scene.

The show then flashes to the present day. The detectives of the Homicide Division of the Philadelphia Police Department are told that new evidence has turned up in an old case gone cold in several cases:

-New evidence has come to light;
-The victim’s physical remains have been discovered;
-A witness has decided to come forward, etc.

All or any of these cause the detectives to give the cold case a new look and begin researching the victim and interviewing their friends, acquaintances, and family.

The friends, family, co-workers, etc. who were introduced in the flashback at the beginning of the episode have aged (in one case, a six-year-old witness to the crime is interviewed by the detectives when she is 94). Gradually, the detectives gather enough evidence to determine the killer, who most of the time is then arrested.

At the end of each episode, the detectives mark the case solved, while an apparition of the murdered person looks on.

The problems in the detectives' personal lives are also featured, though the main emphasis of every story is on the victim and the search for the killer. Most notably, Detective Lilly Rush grew up in a severely dysfunctional, poverty-stricken home with an alcoholic mother.

01. Look Again

In the pilot episode, Lilly investigates a 1976 case involving a teenage girl who was beaten to death on a tennis court during a party at a plush estate.

02. Gleen

The fiancé of a fireman asks Lilly to help prove that a convicted felon, about to be paroled on an unrelated crime, is responsible for the murder of the fireman's first wife.

03. Useless Cats

A serial rapist sends a letter to the squad announcing his return to Philadelphia after five years and his plan to strike again.

04. Churchgoing People

Lilly re-investigates the case of a murdered church organist, when his Alzheimer's stricken widow begins having flashbacks of the night in question.

05. The Runner

After a drug addict brings in an audio tape she found on which a fatal shooting is heard, Lilly reopens a 1973 murder case involving the death of a 21-year-old rookie cop.

06. Love Conquers Al

Lily believes the wrong man is currently serving time for the murder of a 16-year-old girl, after a prison inmate relates a conversation he overheard.

07. A Time to Hate

Rush and Vera investigate a cold case from 40 years ago involving the death of a gay colleage student.

08. Fly Away

After a woman emerges from a two-year coma with no memory of her identity or her daughter's death, the detectives investigate the fall in which the little girl died.

09. Sherry Darlin'

Lilly gets an anonymous phone call from a man claiming he killed an elderly woman back in 1989 and buried her body in the basement of a house. When Lilly checks the dwelling, a body is recovered, but the alleged murderer refuses to identify himself.

10. The Hitchhiker

When the team re-opens the case of a hitchhiker found murdered in 1997 near Atlantic City, they uncover new evidence that leads them closer to the killer.

11. Hubris

A college professor, who lost everything - his career, his family, his reputation - after being suspected of murdering one of his female students in 1995, offers new information regarding the case that he hopes will clear his name.

12. Glued

Det. Stillman asks Lilly to re-open a case he couldn't solve, wherein an 8-year-old boy, Tim Barnes, was murdered in 1980, and the prime suspects included a catholic preist, 3 glue-sniffing teenagers, one elusive suspect, and the boy's own parents.

13. The Letter

Rush and Valens re-open the case of a 25-year-old black woman, who was murdered in 1939, after the woman's granddaughter comes forward with new information.

14. Boy in the Box

Arnold is “The Boy in the Box," found in 1958 and unidentified until now. The case turns ugly when it is uncovered that medical experiments were run on orphans, and by nuns.

15. Disco Inferno

A dance contest ends in murder and a fire in 1978.
Rush and Valens' discovery of a body with several bullet holes in a burned-out disco destroyed in a 1978 fire leads to them to discover the victim's death was not caused by the blaze that claimed other lives.

16. Volunteers

The slain bodies of a white woman and a black man who both went missing in 1969 are discovered wrapped in plastic beneath a building's foundation at a construction site. The investigation reveals the two were involved with an underground abortion clinic and may have been targets of the FBI.

17. The Lost Soul of Herman Lester

The 1987 murder of a high-school basketball player is reopened after the victim's son receives a phone threat that he'll be killed just like his dad was if he plays in the state-championship game.

18. Resolutions

A drunken driver's hit-and-run confession leads the detectives to discover that foul play, not an accident, may have killed the victim.

19. Late Returns

A current murder case leads Rush and Valens to re-open a cold case from 1992 involving a young woman killed after leaving a political event on election night.

20. Greed

Rush and Valens reopen the 1985 murder of Charles Danville, a wealthy stock

01. The Badlands

Lilly returns to her first case when she re-investigates a brutal triple homicide.

02. Factory Girls

Lilly and the team reopen a case from 1943 involving the suspicious death of a World War II female factory worker.

03. Daniela

The discovery of a home movie starring a teenage runaway causes Lilly and Valens to reopen a murder investigation from l979.

04. The House

The night of Johnny Cash's famous Folsom Prison Concert is the scene of a murder that Lilly and the team re-investigate.

05. Who´s Your Daddy?

Lilly reinvestigates the murder of two Cambodian immigrants when their teenage daughter resurfaces with new evidence.

06. The Sleepover

Lilly re-opens a murder from a teenage slumber party in 1989 when a copycat crime occurs years later.

07. It´s Raining Men

A long-term AIDS survivor tries to solve his dead partner's murder. A man who first contacted the disease in 1983 asks Rush and Valens to re-open the unsolved strangulation death of his former partner.

08. Red Glare

When an emotionally scarred son learns of his father's unsolved and brutal murder in 1953, he asks Rush and Valens to re-open the case, as he believes that his estranged mother is the killer.

09. Mind Hunters

When the decomposed remains of a decapitated female body are found in a remote wildlife preserve, Lilly and the team re-open a case which was filed in the Missing Persons Division in 1985.

10. Discretion

Lilly re-investigates the unsolved murder of a prominent assistant district attorney when he is posthumously linked to a scandal.

11. Blank Generation

A cult member's suicide in l978 is re-examined by Rush in light of a new fatality.

12. Yo, Adrian

A boxing referee makes a dying confession about a fatal 1976 fight.

13. Time to Crime

The accidental drive-by shooting of a little girl is re-examined in light of the murder weapon turning up.

14 Revolution

Lilly reinvestigates the murder of Ellie McCormick, a young woman found murdered in her lover's apartment.

15. Wishing

The unsolved 1993 death of a mentally disabled teen, Colin, is re-opened after drawings recreating the boy's murder are found near his grave.

16. Revenge

The unsolved 1998 death of a 9-year-old boy, Kyle, is re-opened after Stillman's priest reveals that a patron confessed to his involvement in the boy's kidnapping.

17. Schadenfreude

Rush and the team re-open the 1984 murder of a rich Philadelphia woman, Lindsay, when her rare ring is found on the finger of a dead junkie.

18. Ravaged

The case of an "accidental" death of a young alcoholic woman, Sloane, is re-opened when her sister claims she might have been murdered by members of a fraternity.

19. Strange Fruit

Still haunted by a childhood memory, Jeffries prompts the team to re-open the 1963 murder of a 15-year-old black boy, Zeke, found beaten to death in a schoolyard.

20. Kensington

The murder case of a young mill worker is re-opened when Rush learns that a recent parolee admitted to stealing money off the victim's body.

21. Creatures of the Night

The unsolved 1977 case involving the death of a Philadelphia doorman, Mike, is given a second look when Rush and Valens try to link his murder to a serial killer about to be released from prison on a legal technicality.

22. Best Friends

Rush and Valens re-open the 1932 case when a truck containing human bones is pulled from the Delaware River.

23. The Woods

The discovery of nine human skulls leads Rush back to George Marks, the serial killer she was unable to incriminate months earlier, and who walked away a free man. As the detectives reinvestigate his mother's murder from 1972, George is forced to emerge from hiding to face Rush again. This time, their very lives are at stake in their final showdown.