Set in the Los Angeles Police Department's Central Division, Adam-12 follows police officers Pete Malloy (Martin Milner) and Jim Reed (Kent McCord) as they patrol Los Angeles. The plots of most episodes follow Malloy and Reed as they handle the various calls for service that they are assigned to or come across, ranging from intense incidents such as pursuits, standoffs, shootouts, hostage-takings, gang violence, terrorism, and undercover assignments, to the mundane (and far more common) routine happenings like traffic stops, disputes, disturbances, narcotics crimes, DUI arrests, fights, and thefts. The series is set in a time before mobile radios were widespread with law enforcement officers, resulting in Malloy and Reed using available private or public telephones to call assistance when they are away from their patrol car.

At the start of the series, Malloy, seeking to resign after the death of his previous partner, is assigned to field train Reed, an inexperienced rookie. After Reed disobeys Malloy's orders but singlehandedly arrests a group of armed suspects during a high-risk shooting call, Malloy sees potential in Reed and decides to remain on the force to guide him through his nine-month probationary period. The first and second seasons are not chronological, with Reed's stated time in the LAPD varying in each episode. Starting with the third season, the series was organized chronologically, and Reed completed his probationary period, with him and Malloy remaining partners. Throughout the series, and especially in later seasons, Malloy and Reed began patrolling other divisions and working on different assignments (such as divisions assigned to patrol neighborhoods like Venice and Hollywood, airport detail, Metropolitan Division, Air Support Division, and even desk duty), occasionally explained as them filling in for other officers or being part of police experience programs. Malloy sometimes fills in for Sergeant William MacDonald as the watch commander in the show's final season. At the same time, Reed finds himself partnered with probationary officers, mirroring Malloy's original role as his field training officer. In the series finale, Reed receives the Medal of Valor for saving Malloy's life and completes an examination to become a detective, while Malloy is set for a promotion to sergeant.

The personal lives of Malloy and Reed came up on occasion and were often tied into their duties, though they rarely extended past conversations, episode subplots, or brief scenes. Malloy is a bachelor who has at least five girlfriends (not simultaneously) over the course of the series. Reed is married to a woman named Jean (played by several actresses, including Mikki Jamison and Kristin Nelson); in the second season, they are shown to have a son, Jim Jr. (or Jimmy). Occasionally, Malloy and Reed are depicted socializing outsi…

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• Martin Milner as Officer Pete Malloy: A veteran police officer who initially plans to resign from the LAPD, but remains on the force to guide Reed through his probationary period. He is acquainted or familiar with many officers in Central Division, as well as some citizens in Adam-12's usual patrol areas. By season 7, he is preparing for a promotion to Sergeant and spends time working as a watch commander. He is single but seeking, and the subplots of several episodes revolve around his dating life. His badge number is 744.
• Kent McCord as Officer Jim Reed: A rookie police officer assigned Malloy as his field training officer. After his probationary period, he remains Malloy's partner. Malloy does not trust him to drive the cruiser, a recurring joke throughout the series. At the end of season 7, he applies to become a detective. He has a wife, Jean, and a son, Jimmy. His badge number is 2430.
• Shaaron Claridge as herself: A police dispatcher and desk clerk. Working alongside an unnamed male dispatcher, Claridge appears in voice only in almost every episode. She only physically appears briefly in season 5, episode 20. Claridge was an actual LAPD dispatcher who acted as a side job until her retirement in 1990.
• William Boyett as Sergeant William "Mac" MacDonald: The watch commander of Central Division. He is responsible for commanding and organizing the officers under him, including Malloy and Reed, who report to him and are sometimes assigned to cases by him. Mac is issued a station wagon and a Federal Riot Gun capable of firing tear gas rounds, which he and other officers are authorized to use in standoffs. When he goes into the field, his call sign is 1-L-20. He is mentioned to have a wife named Mary, a son named William "Billy", a daughter named Elizabeth, and a brother named Bryan.
• Gary Crosby as Officer Ed Wells: A police officer and one of Malloy's friends. In his first appearance in season 1, he is depicted as very hot-headed and reckless, though this behavior is toned down in later seasons, with him mostly serving as comic relief instead. Though he acts cocky and makes jokes at the expense of other officers, he is also fairly dimwitted, and several of his jokes and actions fall flat or backfire. At least once, his cockiness landed him in the hospital with gunshot wounds, and Reed and Malloy had to come to his aid. Throughout the series, he is seen with numerous different assigned partners. He was proposed as a potential successor to Malloy for a planned eighth season that ultimately never materialized.
• William Elliott as Motor Officer Gus Grant: A motorcycle officer introduced in later seasons. He is laid-back but dedicated to his work. His use of a motorcycle is a factor in several incidents, with Adam-12 usually called in to support him with their cruiser. He is one of the only African American characters in the series to have a recurring role.

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Universal Studios co-produced the show with Mark VII Limited. The series' first episode, "Log 1: The Impossible Mission", was filmed in September 1967, a year before the pilot was picked up.

The production of Adam-12 involved showing all aspects of correct police procedures. Police cruisers played a central role, as "Webb wanted the vehicle itself to be considered a character". The show's routine use of police radio communications and jargon helped reinforce "the sound of radio as an anti-crime technology." The police vehicles used in the production of the show were purchased from local dealerships and outfitted by the prop department to accurate LAPD cruiser specifications.

The driving scenes were filmed on public streets using a dedicated camera platform mounted to the cruiser's hood, which was towed by a station wagon. The platform held three cameras: the center camera would film both Milner and McCord in one shot, while the left and right cameras would film Milner and McCord, respectively, on a cross-angle. All three cameras were remote-controlled and would roll simultaneously. For the pilot, Webb did not like the reflections of the sky on the windshield (which made Milner and McCord difficult to see), so the windshield of the vehicle was removed; however, this made the actors' hair blow and caused issues with the audio recording equipment. The prop department's solution was to reinstall the windshield and build an overhead shield that extended over the hood and windshield of the car to block the reflections. Sides were added to create a large "black box" over the front of the vehicle with both ends "open", which allowed for more controlled filming circumstances. During filming, the script supervisor would lie on the floor in the back of the car to read script dialogue for the voices coming from the police radio, which Milner and McCord would respond to. The director and the sound man would sit in a rear-facing seat in the back of the station wagon towing the police car.

Most scenes in the older seasons were filmed at the Universal Studios Lot, mostly in Courthouse Square and Colonial Street. Starting with the third and fourth seasons, the series gradually shifted to filming in actual locations in Los Angeles and the San Fernando Valley. The garage used tow trucks from the LAPD's North Hollywood Division that was located near Universal Studios.

The police station used in the series was the Rampart Police Station, located at 2710 West Temple Street. The station was built in 1966, two years before the series began, and could thus be considered "state of the art" for most of the series' run. Rampart Police Station was closed in 2008, and Rampart Division moved to the newer and larger Rampart Community Police Station. The old station was renovated to serve as the headquarters for the LAPD Metropolitan Division. The renovated building, now known as the LAPD Metropolitan Division Facility, opened in 2016.

In 1974, during the production of the seventh season, Martin Milner signed to play Karl Robinson on the short-lived television series The Swiss Family Robinson, taking him off Adam-12. Several options were tested during this season, which would allow the series to continue should the production of a p…

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