Law & Order episodes are typically segmented into two parts, roughly at the halfway point; the first part follows police and detective work, and the second follows the legal and courtroom proceedings of the case. The show dwells little on the characters' back-stories or social lives, focusing mainly on their lives at work.
For most of Law & Order's run, the cold open or lead-in of the show began with the discovery of a crime, usually a murder. The scene typically began with a slice of everyday life in New York City. Some civilians would then discover the crime victim, or sometimes the crime would occur in a public place and they would be a witnesses or a victim of a crime. The only exception to this are in the early seasons, mostly Seasons 1 & 2, the crime would usually be discovered by a pair of patrol officers or beat cops or in later seasons when the cold open was replaced with rapid cuts of the victim's final moments, similar to Law & Order: Criminal Intent.
The police are represented in the show by the New York City Police Department 27th Precinct homicide department (in the show it is common that the detectives also investigate other cases, like kidnappings and rape, but in the real world these cases are handled by other units and divisions), with two homicide detectives, a senior detective (usually a veteran cop) and a junior detective (usually a young but capable detective), who report directly to their boss at their precinct either a Lieutenant or a Captain depending on rank of the Precinct Commander. During the preliminary crime scene examination, the featured detectives make their first observations and will come up with theories followed by a witticism or two, before the title sequence begins.
The detectives often have few or no good cluesthey might not even know the victim's identityand must usually chase several dead ends before finding a likely suspect(s).
They start their investigations at the crime scene by talking to any witnesses at the scene whilst the Crime Scene Unit (CSU) forensic technicians start processing the scene by collecting the necessary evidence. The CSU personnel have many tools at their disposal including the materials needed to develop fingerprints, cast footwear and tire impressions, follow the trajectory of bullets fired through windows and the chemicals necessary to observe blood under special lighting conditions that would otherwise be invisible to the naked eye, all of which might provide clues to help the police later. The Medical examiner (M.E.)'s office will also be shown to collect the body from the crime scene.
Later the medical examiner will perform an autopsy on the victim, offering more clues to the victim's cause and time of death (sometimes obtaining the victim's identity from dental records or fingerprints) which the detectives will read about in the M.E.'s autopsy report and by talking to the M.E. who performed it. When the detectives know the victim's identity they will inform their relatives or loved ones of their death and attempt to get more information on the victim's life and possible suspects.
The detectives continue their investigation by interviewing witnesses and possible suspects, all the while tracing the victim's last known movements and victim's state of mind (by talking to the victim's family, friends and co-workers). Sometimes they will have someone they suspect of the crime and will trace their last known movements and state of mind by talking to the people in the suspect's life, until they are either ruled out or dead certain of their guilt. They also visit the crime laboratory to submit and view evidence (e.g. fingerprints, DNA and ballistics, etc.), they may also look into any background information such as financial details and criminal history on both the victim and lead suspect. In some instances, psychologists and/or psychiatrists are called in for insight into the criminal's behaviour or modus operandi. All the while, the detectives report to their commanding officer, keeping them informed and being advised on how best to proceed next.
When the detectives are certain they have the right suspect(s), the police will take the case to their boss, who decides if there is enough for a search and/or arrest warrants (though sometimes the commanding officer will consult with the New York City District Attorney's office to see if the case is strong enough) and whether or not any back-up (such as uniformed officers or armed tactical team) is needed. The detectives will then arrest the suspects(s), with sometimes the police having to chase the accused through the streets of New York. The scene then shifts to the interrogation room where the detectives interrogate the suspect(s), until they ask for a lawyer, their defense attorney shows up and asks the suspect not to talk anymore or the Assistant District Attorney from the D.A.'s office decides they have enough to press charges.
Towards the middle of a show, the police will begin to work with the prosecutors to make the arrest, though sometimes the Assistant District Attorney will appear earlier to arrange a plea-for-information deal or to decide if the detectives have enough evidence for search warrant(s) and/or arrest warrant(s) before arresting the suspect or suspects. An arraignment court scene will follow, in which the defendant(s) plead (usually not guilty) and bail conditions are set.
The matter is then taken over by a pair of representatives from the New York County District Attorney's Office, an Executive District Attorney and an Assistant District Attorney. They discuss deals, prepare the witnesses and evidence, and conduct the People's case in the trial. The District Attorneys work together and with the Medical Examiner's office, the crime laboratory (including fingerprint analysts, DNA profilers and ballistics analysts), and psychologists and/or psychiatrists (if the defendant uses an insanity plea), all of whom may be needed to testify in court for the prosecution. The police may also reappear to testify in court or to arrest another suspect, but most investigation in the second segment is done by the D.A.'s office, in consultation with the District Attorney for advice on the case, as the D.A., being an elected official, sometimes brings political considerations to bear concerning decisions to prosecute various alleged offenders. If the case is very weak then the police would re-investigate.
Unlike many other legal dramas (e.g. The Defenders, Matlock, Perry Mason and L.A. Law), the court proceedings are shown from the prosecution's point of view, with the regular characters trying to prove the defendant's guilt, not innocence. After the arraignment of defendants the D.A.s proceed to trial preparation, including legal research and plea negotiations. Some episodes include legal proceedings beyond the testimony of witnesses, including motion hearings, (often concerning admissibility of evidence); jury selection; and allocutions, usually as a result of plea bargains. Many episodes employ motions to suppress evidence as a plot device, and most of these end with evidence or statements being suppressed, often on a technicality. This usually begins with the service of the motion to the D.A. team, follows with argument and case citations of precedent before a judge in court, and concludes with visual reaction of the winning or losing attorney.
Many episodes use outlandish defense scenarios such as Diminished responsibility (e.g. "Genetics"/"Television"/"God"/"the devil made me do it" and intoxication defence) and temporary insanity (e.g. "Black Rage"/"White Rage"/"Sports Rage"). Some episodes revolve around moral and ethical debates including the right to die (euthanasia), the right to life (abortion), and the right to bear arms (gun control). Episodes usually ends with the verdict being read by the jury foreperson and a shot of both the winning and losing parties. The scene then shifts to the District Attorney's office, where the team is leaving the office to go home while contemplating either the true guilt of the accused, the defense scenarios that were used, or the moral or ethical issue that was central to the episode.