Seasons 1–3: Brenda Leigh Johnson arrives at the LAPD facing active mutiny orchestrated by Captain Russell Taylor, who leads the squad in filing mass transfer requests. Through diagnostic investigative skill, Brenda gradually earns the trust of Sergeant Gabriel and veterans Provenza and Flynn. Season 2 reveals Brenda's CIA interrogation background and clandestine national security clearances in "Serving the King", while deepening her relationship with FBI Agent Fritz Howard as he works through recovery from alcoholism. In Season 3, personal upheaval follows visits from Brenda's parents, culminating in Fritz and Brenda moving in together and becoming engaged, despite Brenda's work addiction.
Seasons 4–6: Reorganized into the Major Crimes Division, the team encounters defense attorney and sociopathic serial predator Phillip Stroh, who eludes prosecution and becomes Brenda's primary nemesis. In Season 5, Captain Sharon Raydor from Internal Affairs begins auditing Brenda's unit, while Brenda endures the physical decline and death of her cat, Kitty, and marries Fritz. In Season 6, administrative conflict erupts during the race for Chief of Police. While interrogating gang hitman Terrell Baylor, Brenda drops him off in rival gang territory after his confession, where Baylor is promptly executed—a severe breach of ethics that triggers the final arc.
Season 7 and Finale: Civil attorney Peter Goldman files a federal wrongful death lawsuit on behalf of the Baylor family against Brenda and the LAPD, aided by confidential squad leaks. Brenda and Raydor eventually discover the unwitting leak was Sergeant Gabriel, whose girlfriend Ann Mason was covertly duplicating files for Goldman. Brenda is dealt another devastating blow when her mother, Willie Rae, dies suddenly in her sleep. In "The Last Word", Brenda targets Phillip Stroh using runaway teen Rusty Beck as an eyewitness. Desperate to secure proof, Brenda physically assaults Stroh in an elevator for a DNA sample, triggering her suspension. Stroh later invades Brenda's home; she shoots and wounds him before the squad arrives to arrest him. Acknowledging that she can no longer navigate LAPD politics, Brenda leaves to become Chief of the District Attorney's Bureau of Investigation, leaving the division to Sharon Raydor and parting with Provenza's heartfelt words, "It looks like love."