Mary Richards, a single native Minnesotan moves to Minneapolis in 1970 at age 30 and becomes Associate Producer of WJM-TV's Six O'clock News. Her sincere, kind demeanor often acts as a foil for the personalities of her co-workers and friends.
After arriving in Minneapolis, Mary leases an apartment in a house from her friend, Phyllis Lindstrom. Also leasing an attic loft from Phyllis is Rhoda Morgenstern, with whom Mary becomes fast friends. Mary also bonds with Phyllis's precocious daughter, Bess.
Mary applies for a secretarial job at television station WJM-TV, the area's lowest rated station. After meeting with news producer Lou Grant, she is informed that the position has been filled but she is hired as an Associate Producer. Later, Mary is promoted to News Producer when Lou becomes the station's news director. While at WJM, she quickly becomes friends with news writer Murray Slaughter and the vain (and incompetent) news anchor, Ted Baxter. Within the office, Mary is often the voice of reason. Lou, who is always referred to by Mary as "Mr. Grant", later develops an adoration and almost-fatherly relationship with Mary.
Other friends of Mary's include Sue Ann Nivens, host of The Happy Homemaker at WJM, and Georgette Franklin, who later marries Ted.
In the final episode of the series, the entire newsroom staff lose their jobs in an effort to boost sagging ratings. Ted, ironically, keeps his job, despite being the primary cause for the low ratings.