The 20th and final season of Law & Order premiered on NBC on September 25, 2009, for which it was moved from its previous time slot to air on Fridays at 8 p.m. ET for the NBC broadcast. L&O was moved to Monday nights on March 1, 2010 with a two-hour telecast at 9 p.m. ET, before settling into its new time slot at 10 p.m. the following week, March 8.
During the season, The Jay Leno Show in primetime was cancelled and Leno returned to the 11:35 pm Tonight Show because of affiliate dissatisfaction. Three more episodes of Law & Order were ordered to fill the primetime gap, increasing the number of episodes for the season to 23.
In early January 2010, NBC initially announced they were going to renew Law & Order a 21st season. NBC's President of Primetime Entertainment at the time, Angela Bromstad, said, "I’m a "Law & Order" junkie... I wouldn’t want to be responsible for not having "Law & Order" break the record." But on May 14, 2010, NBC abruptly cancelled Law & Order after 20 seasons on the air, tying it with Gunsmoke as American network television's longest-running, regularly scheduled primetime drama. The final episode aired on NBC May 24, 2010.
The late announcement of the cancellation resulted in Law & Order not having a "series finale" episode. Had the series continued, it would have been the last to feature S. Epatha Merkerson, who prior to the cancellation announcement had herself announced that she would not return for the 21st season.
The body of a young war veteran is found in a parking garage and the murder is connected to a law professor who used to work for the Department of Justice.
A crime scene investigator is murdered in her apartment. Her fiance is suspected, but then a journalist is attacked and DNA is found at the scene that links this crime to the murder.
Detectives Lupo and Bernard investigate when an aspiring musician is found dead. A bag of cash leads them to multiple suspects.
A mother of ten adopted special needs children is murdered. The day after her murder, her husband appears on reality TV.
A doctor who performs late-term abortions is murdered in church. The case falls apart for Cutter when Rubirosa is touched by an abortion nurse's testimony.
Bernard becomes the target of a dangerous website that is partly responsible for a homicide the detectives are investigating
When a young woman is found dead in her apartment, a few tell-tale signs quickly lead Detectives Cyrus Lupo (Jeremy Sisto) and Kevin Bernard (Anthony Anderson) to believe that the killer was not after her wealth.
A deadly drunk-driving car accident brings Detectives Cyrus Lupo and Kevin Bernard to the scene when a suspicious nasal spray is found. After the medical examiner realizes the nasal spray is a powerful anesthetic that could easily disorient its user, the detectives become suspicious of the victim's boss, Zach Marshall
A case that begins as a search for a missing witness spirals into a murder conspiracy with ADA Connie Rubirosa caught in the middle.
When an elderly man defends his shop from an armed robbery, Detectives Kevin Bernard and Cyrus Lupo suspect that this incident could have been an inside job. After further investigation, the detectives discover that the mugging might have been a scare tactic for a shakedown.
A community organizer who was working on voter registration is found dead with the word "FED" written on his chest. The detectives initially suspect an anti-government motive for the murder, but the case gets more complicated when they learn that the victim was secretly gathering incriminating information about the organization he worked for.
A freelance journalist is murdered. The detectives find a connection between her and a talk show host who has had a series of workplace affairs.
When a family of four is found murdered in their home, Detectives Cyrus Lupo (Jeremy Sisto) and Kevin Bernard (Anthony Anderson) discover that the deaths may be related to the deceased teenage daughters troubled friend, Bonnie.
When detectives Cyrus Lupo (Jeremy Sisto) and Kevin Bernard (Anthony Anderson) come upon the burning body of Cesar (guest star Noel Rodriguez), a sixteen-year-old charter school student from a bad neighborhood, they begin to question his young mentee Moses (guest star Aaron Shaw), who was the last person to see him alive.
A young woman is found murdered in a hotel and the evidence leads to a medical student.
A man is convicted of a hate crime for allegedly killing a gay man, but a group called the Innocence Coalition is determined to clear his name.
The debate rages over how to treat an informant who shot and killed four police officers. However, did one of the officers try to put undue pressure on him to help catch a dangerous criminal?
When an environmental scientist, Dr. Oscar Silva, is poisoned during a global warming symposium, Detectives Cyrus Lupo and Kevin Bernard initially suspect competitors from Dr. Silva's field of study. It is soon discovered that Dr. Silva is involved in a messy custody battle with his wife’s ex-husband, Phillip Shoemaker and the focus of the investigation quickly shifts to the victim’s family. Meanwhile, the case becomes personal for Cutter as it stirs up unpleasant memories from his own past.
When the body of a model is found burning in an alley, Detectives Lupo and Bernard search for her killer.
A man's double life is discovered after he is stabbed to death.
A reality TV crew finds a dead body locked in a dog cage.