The detectives have to determine if the death of a boy was caused by the intentional destruction of a contractor's building at his own hands, or that he was the intended target.
A young boy climbing a building under construction for fun dies when a bomb destroys it. The building falls onto the trailer of the general contractor.
The detectives find that contractor was having severe financial difficulties. The contract for the building stipulated that he would be released from all obligations in the event of a terrorist act. Furthermore, he had access to explosives and lived at the construction site. He was subsequently arrested, albeit on circumstantial evidence.
Further investigation reveals the contractor was having an affair with a married woman, and that her husband was also a demolitions expert. When the detectives uncover detonation cord matching the cord found at the scene, the prosecution decides to prosecute the husband instead.
After a quick trial, the husband is convicted of arson and murder and is sentenced to prison. After the verdict, Lennie Briscoe shows up at the District Attorney's office and shares some concerns he has after his cross examination at the trial. He realized that the contractor's alibi, that he had been taking a walk at the time of the explosion, was belied by the fact that he did not arrive back at the scene until 6:30, long after the time of the explosion and when he should have returned if he lived there.
The prosecutors get a warrant to tap the phone of the wife, whereupon they record a conversation between her and the contractor that seems to imply a plot between them to frame the husband. The state attempts to bring charges against them, but because someone has already been convicted of the crime, a judge finds the warrant faulty and does not allow the taped conversation to come into evidence.
When the prosecution cannot get the husband out of jail without convicting someone else of the crime, they change their theory of what the tape means even though they do not believe it: instead, they claim that the tape implies a plot between the husband and wife. This gets around the problems with the warrant. The husband, eager to get revenge on his wife, is happy to testify against her even though they did not have a plot together. The wife pleas to a reduced sentence in return for the truth, implicating the contractor and clearing her husband, who is subsequently released.