Lilly investigates a 1995 shooting spree at a mall in which two teen boys killed 15 people before committing suicide. The case is reopened when a video of the crimes is found and it suggests that a third shooter was involved. Elsewhere, Stillman asks Valens to investigate whether Valens' brother may have been molested by a boxing coach in the 1980s.

The team re-opens the 2004 case of a female veteran of the Iraqi War when her prosthetic arm is found in a river. Meanwhile, Scotty visits his older brother in connection with a case that brings up painful memories from their childhood.

When the remains of a sandhog miner who disappeared in 1947 are found in a subway service tunnel, the team is prompted to re-open the case when they suspect the victim might have been murdered for being a union activist.

A 6-month-old girl's 1982 death, originally ruled to be a result of sudden-infant-death syndrome, is investigated when new evidence reveals the baby may have drowned. Also, Lilly learns that Ray is in the hospital after apparently being beaten.

Lilly re-opens a 2003 case in which an autistic boy's parents were shot in their car when Joseph brings her information that the boy might have seen the killer.

The apparent 1958 suicide of a popular rock-and-roll DJ is reinvestigated after a documentary filmmaker discovers a tape recording made at the time of the victim's death by gunshot at his radio station. Also, Valens tries to get his brother to testify against a suspected pedophile.

The 1979 murder of a schoolteacher who was stabbed to death in a wooded area is reopened when the victim's bloodstained jacket is found stuffed in a tree trunk with a happy-face key chain in a pocket. The case was originally investigated by Jeffries when he was a rookie detective.

The 1975 disappearance of an 8-year-old girl who was never found is reinvestigated after a note written by the girl shortly before she vanished turns up. Their friendship rattled many in the predominantly white neighborhood and the girls quickly became targets of hatred.

A homely 33-year-old woman is gunned down in an alley in 1989 and her unsolved murder is reinvestigated when a tape of the victim, made for a dating service, turns up in a suicide victim's apartment. Apparently, it was what the man was watching when he killed himself. Also, Vera moves into an apartment of his own

The team re-investigates the 1968 death of a policeman who was shot in his patrol car. The case was then ruled as a drug bust gone bad, but new evidence now reveals the victim may have been romantically involved with his partner.

Lilly and Valens travel to Nashville and Knoxville to investigate the 2000 murder of a country-music singer who was gunned down outside of a Philadelphia bar where he and his band had recently played. The unsolved case is reopened when the gun used in the homicide turns up.

Lilly investigates the murder of a 17-year-old prep-school student who disappeared in early 2006 and whose body is found stuffed in an oil drum outside an abandoned warehouse in a drug-infested part of the city. Also, Vera pays a parking ticket for his neighbor and later asks her out on a date.

Lilly investigates the 1996 murder of a wealthy divorced woman who was found dead in her ex-husband's indoor swimming pool. The original investigation ruled the death accidental, but new evidence suggests it was a homicide.

Lilly investigates the deaths of a married couple who perished in 1981 when a supposedly accidental gas-leak explosion set their house on fire. The case is reopened when fragments of a bomb are discovered at the house during renovations. Back at headquarters, Stillman takes care of his 4-year-old grandson.

Lilly investigates the 1964 murder of a 17-year-old girl who was sent by her parents to a home for unwed mothers, where she gave birth to a girl. The next day the teen was killed. The case is reopened when the victim's daughter comes forward, having just learned that she was adopted and that her biological mother was slain.

When a body is found in a dumpster, the team re-opens the 1984 case of a murdered teenage boy who wanted to become a dancer against his father's wishes.

The 1999 murder of a homeless woman, whose body is found at the bottom of a river in a station wagon, is investigated.

A father pushes a registered sex offender off of a building and threatens to kill a different sex offender each day until the cold-case unit solves his son's 1987 rape and murder.

The 1997 murder of a 16-year-old girl high-school cheerleader is investigated after an anonymous confession is discovered in a modern-art exhibit.

The team re-investigates their oldest cold case yet, the 1919 murder of a young woman who was a passionate advocate for women's right to vote.

The 2005 murder of a longshoreman who accidentally became involved in a human-trafficking ring run by the Russian mob.

The 1998 death of a terminally ill man who died months before he was expected to is investigated after a nurse confesses to six so-called mercy killings.

In the fourth season finale, a hostage situation arises when the entire team is held captive while they're investigating the 2006 killings of a family who had just moved into a new house.