Part 1

Christmas 1924: Two-year-old Frances Gumm performs in public for the first time, singing "Jingle Bells" in Grand Rapids, Minnesota. Her mother, Ethel, watches from the audience while her father, Frank, watches from backstage. Ethel is unhappy with her marriage because of Frank's romantic interest in men. To help herself cope, she moves her family to Hollywood with the hope that her daughters will break into the movie business.

1935: Frank takes Frances, now using her stage name of "Judy Garland," to the studios of Metro Goldwyn Mayer to audition. MGM chief Louis B. Mayer is not impressed with her rendition of "Zing! Went the Strings of My Heart', but when she sings a different song an impressed Mayer says, "Little girl. Big voice." Thirteen-year-old Judy (played by Tammy Blanchard) signs an MGM contract but, because of her age, they do not know what to do with her and keep giving her radio appearances. Tragedy strikes one night when she is told her father has been rushed to the hospital. She is also told that the doctors have put a radio beside his bed, so he will be listening. While her sisters, Suzy and Jimmie, are in tears over their ill father, Ethel shows no emotion at all. He dies the next day.

1938-1939: Judy's movie career is now blooming. Now sixteen, she finds herself in competition with MGM's new glamorous star, Lana Turner, who is everything she is not: tall, thin, and blond. Judy also becomes jealous as Lana steals everybody's, including Mickey Rooney's, attention on her birthday. MGM purchases the rights to L. Frank Baum's classic children's book, The Wonderful Wizard of Oz. Rumors spread that Shirley Temple might be playing Dorothy, but when 20th Century Fox refuses to lend her out to them, Judy is cast. She is prescribed some pills to help her sleep and to give her energy to work, and she is also forced to lose weight. She is then seen filming the "Yellow Brick Road" sequence with the Scarecrow, Tin Man, and Cowardly Lion. On the first take, they all close in and shut her out, prompting director Victor Fleming to yell, "You three dirty hams! Let that little girl in there!" The film turns out to be a huge success and she is catapulted to international stardom.

Early 1940s: Judy begins a romance with bandleader Artie Shaw, who has already been married twice. This causes much concern, especially for Ethel, who has now remarried. Judy continues to see him and is shocked when he elopes with Lana Turner, leaving her heartbroken and reluctant to return to the studio since she feels she has to compete with all the goddesses. While filming the "I Got Rhythm" sequence for Girl Crazy, she is continually being reprimanded by the director, the no-nonsense Busby Berkeley, over not putting enough energy into her performance. Eventually, she collapses on the set and is granted three weeks rest, despite the doctor's instruction that she needs six. Aged just nineteen, she marries composer David Rose, but the marriage lasts only nine months.

1944: Judy (now played by Judy Davis) meets Vincente Minnelli (Hugh Laurie), who is the director of her next film, Meet Me in St. Louis. She is then shown filming "The Trolley Song" sequence. She and Vincente marry in 1945. On their honeymoon, she tells him she plans to quit MGM when her contract expires and that she is pregnant. She then throws away a bottle of her pills and vows never to take them again.

1947: Now mother to Liza, Judy is forced to renew her contract with MGM. While filming The Pirate, she has a mental breakdown and Vincente finds out she's taking the pills again. The marriage spirals downward from there.

1950: Judy is suspended from MGM and from filming Annie Get Your Gun (she was replaced with Betty Hutton). She also tries to commit suicide by slashing her throat with a broken glass. She is fired by MGM and her marriage to Vincente falls apart due to his exhaustion at her mood swings. During this time, she meets Sid Luft (Victor Garber). He helps her with her show business comeback at the Palace Theatre on Broadway.

Part 2

Early 50s: Judy marries Sid in 1952 and a few months later she gives birth to her second child, Lorna. In 1953, Ethel dies in a parking lot after suffering a heart attack. Initially, Judy does not react to the news, having been estranged from her for years, but while filming "The Man That Got Away" sequence for A Star Is Born, her first film since MGM fired her, she misses her mark, and starts crying in her dressing room, not exactly sure if she is upset over Ethel's death. She receives an Academy Award nomination for her performance. However, shortly after the film's initial release, Sid is enraged to see some of their best scenes (and important ones too) have been cut under orders from distributors who felt the film was too long, thus killing both the flow of the story as well as its box office potential. In 1955, a day before the 27th Academy Awards, Judy gives birth to her third child, Joey, but on the night she loses the award to Grace Kelly, much to the shock and disappointment of her friends, and much of the film world.

Late 1950s: Judy is now struggling with debts, her weight has ballooned, and at this stage, her marriage to Sid is starting to crumble.

Early 1960s: After overcoming a life-threatening illness and slimming down, Judy tours America, the high point being a concert at Carnegie Hall. As her marriage to Sid continues to collapse, she wins custody of their children.

Mid 1960s: Judy gets her own television series, but she is forced to go on the road again after it is cancelled, due to low ratings, for playing in the same time slot as "Bonanza" and for critics at the studio complaining about her touching the other guests on the show, which was a strict taboo at that time on the networks.^[1] Her tour of Australia starts off well, but her concert in Melbourne is a flop; she has trouble remembering the words to her songs, stumbles, and is heckled by the audience before being booed off stage. She later marries for a fourth time, this time to Mark Herron. This marriage lasts a mere five months as he turns out to be gay and is discovered in bed with a male pool cleaner, and she throws him out. Lorna begins to understand the connection between her mother's erratic behaviour and her medication. Judy reconciles with Sid, who books her at the Palace Theatre, this time with Lorna and Joey. He gives Lorna some instructions on how to take care of her mother. However, life with Judy (which included constantly—and secretly—moving from one place to another because of her inability to pay bills) and looking after her and Joey becomes too much for Lorna, who collapses from exhaustion. Fearing for his children's safety, Sid takes them to live with him in Los Angeles.

1969: Judy marries for a fifth and final time. Her new husband is Mickey Deans. They settle in London. Liza, Lorna, and Joey call her on her 47th birthday, and say they will come and spend the summer with her when school finishes in two weeks. Twelve days later, she dies from an accidental overdose of sleeping pills. A hysterical Lorna sobs in Sid's arms. The film ends with Judy performing "Get Happy".