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 witnesses or a victim of a crime. The only exception to this is in the early seasons, mostly Seasons 1 & 2, the crime would usually be discovered by a pair of uniformed officers on patrol 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 other than homicide or attempted homicides like kidnappings and rape, the latter especially in the first nine seasons of the show before Law & Order: Special Victims Unit premiered. However, in the real world, these cases are handled by other units and divisions.
The viewers are introduced to 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). When they first arrive at the crime scene they are met by the first responding officer or a Crime Scene Unit (CSU) forensic technician, who will inform the two lead detectives on everything known at that point. It's during they're preliminary crime scene examination, that the featured detectives will make their first observations and will come up with some theories followed by a witticism or two before the title sequence begins.
The detectives often have few or no good clues-they might not even know the victim's identity-and 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 while the CSU technicians assist them in the processing of the crime scene as well as determining the proper routing of evidence between the Medical Examiner's office, the Crime Lab and the NYPD Property Clerks office. The CSU has many tools at there disposal to process a crime scene 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. The unit is also trained to process a crime scene in a hazardous environment, for example following a nuclear, biological or chemical attack.
The medical examiner (M.E.)'s office will also be shown to collect the body from the crime scene. As well as appear to do an on-site investigation into manner and cause of death; or identification of remains. Later the medical examiner will perform an autopsy on the victim(s), offering more clues to the victim's cause and time of death (sometimes obtaining the victim's identity from dental records or fingerprints and other forensic evidence collected for DNA extraction and typing;) 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 in checking their alibi they will trace the last known movements and the state of mind of the current suspect by talking to the people in the person(s) life until they are either ruled out or dead certain of the guilt of the person they suspect. In later seasons CCTV, GPS and Cell phone tracking might be used to track suspect(s) and victim(s) movements. They may even ask victims and witnesses to look through photographs in mug books or to view police lineups where they will try and identify the suspect(s). If gang or drug connections are suspected the police might talk to other Police units/squad specializing in those types of crimes. They may even approach Criminal Informants to see if they have heard anything on the street about the crime itself.
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 behavior 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 warrant (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 backup (such as uniformed officers or an armed tactical team) is needed. The detectives will then arrest the suspects(s), with the police sometimes having to chase the accused through the streets of New York.
The scene might shift to the interrogation room where the detectives interrogate the suspect(s) until they either confess, 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 or arrest warrants before arresting the suspects. An arraignment court scene will follow, in which the defendants 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 Assistant 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 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 of the investigating in the second segment is done by the D.A.'s office, (even though in real life this would be done by the District Attorney Investigation Unit). They will also consult 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 the 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 allocations, 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 a visual reaction of the winning or losing attorney.
During the trial, both the Prosecutor and lead defense attorney take turns arguing their cases in front of the jury. They both directly examine and cross-examine the witnesses asking them questions that support their arguments in their cases. Objecting when each other goes beyond the scope of what the law will allow eg. "leading/badgering the witness" or "Assuming facts, not in evidence" etc. to which the judge will either sustain or overrule. Many episodes use outlandish defense scenarios such as diminished responsibility (e.g. "Genetics"/"Television"/"God"/"the devil made me do it" and "intoxication defense") 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).
After the trial, the jury will deliberate their verdict and once agreed upon the jury foreperson will read it out to the court. Either verdict will show the reaction of both parties with the guilty verdict showing the defendant being handcuffed by the bailiff and led away to go to prison or if found insane to a secure psychiatric facility. Whilst if found not guilty the defendant will be released and will thank their attorney before joining their family. We will also see the prosecutors look at the family of the victim to see their reaction to the verdict. The scene may 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.