In the intro we can see Lt. Van Buren being ordered by her superiors to raise the homicide clearance rate or she can can end up transferred.
The detectives go through the cold cases and pick up one that seems most likely to dig up something new on. Most of evidence lost, witnesses deceased or unsure what actually happened 20 years ago, they put the case together rather slowly. We suspect, though, that the retired leading detective is not saying everything he knows. It's only at the trial on the stand he decides to tell the whole truth - defendant's influential father had his friends in NYPD sweep the case against his son down. Next day Jack offers the defendant a plea for a manslaughter and it is obvious he accepts it with relief - living 20 years with a burden of murder made him drink and led him several times to therapeutic groups that didn't help. Now he finally finds a closure, as well as victims' mother and the retired detective. The episode finishes with Lennie Briscoe forgiving his old friend.