In Missoula, Montana, in 1924, 19-year-old Alice Monroe works as a librarian. Due to a history of poor health, she is overprotected by her father, Missoula mayor Clark Monroe, and his right-hand man, Murdoch “Mac” MacTavish. Alice has a long-standing crush on Mac, who serves as her driver and bodyguard. One of her duties is delivering books from the Missoula Library to the isolated families in the surrounding mining camps. As she and Mac prepare for a trip, another librarian retrieves a copy of Robert Louis Stevenson’s Treasure Island from the returns pile just in time for Alice to bring it for a boy named Nathaniel, who asked for more adventure stories the last time Alice passed through his town, recognizing the power of stories to bring “delight and adventure” to people living difficult lives. During their drive along the Blackfoot River, Alice feels a growing desire to expand her work and do more for the remote communities. An idea begins to form as a train whistle sounds in the distance.

In 1914, in the Montana mining town of Hell Raisin’ Gulch, 14-year-old Colette Durand is being raised by her father, Claude, a French-Canadian miner. Claude is a union organizer who secretly works to rally his fellow miners against the oppressive Anaconda Copper Mining Company, which has had him beaten for his efforts. Claude has instilled in Colette a deep love for stories, and he often entertains his daughter by reciting Shakespeare’s plays from memory. During their annual camping trip, Claude explains that he loves Shakespeare because the plays were written for common people, offering them a temporary escape from hardship. Colette understands the parallel to their own lives, which are dominated by the grim reality of the mines, and by the campfire she begins reciting A Midsummer Night’s Dream for her father.

Immediately after Millie punches Foxwood Hastings, Katherine Kellock, a high-ranking FWP executive, arrives. She expertly diffuses the situation, sending the shaken secretary home and escorting a furious Hastings out of the building. In her office, Katherine informs Millie that while Hastings is demanding her termination, she will not be fired. Instead, Katherine offers her a new assignment: editor of the FWP’s field office in Missoula, Montana. Katherine reveals that the Missoula staff is on the verge of being dismissed after submitting blank interview forms and incomprehensible essays for their first deadline. Katherine suspects sabotage, theorizing that Montana’s powerful “Copper Kings” may have interfered to prevent any unflattering information from appearing in the state guide. Millie is tasked with investigating the situation and ensuring the office produces quality work by the next deadline; if they don’t succeed, the entire staff will have to be let go, including Millie.

In 1924 Missoula, Alice Monroe has the idea to create a mobile library in a boxcar that can travel along the existing rails to serve remote mining and logging camps. Her father is too preoccupied with is mayoral duties to listen to her idea. On her way to pitch the concept, Sidney Walker—the heir to a local mining fortune, and a man Alice sees as lazy and entitled—slows his car to tease her with sardonic and sexually suggestive remarks. Mac who intervenes, telling him to move along, irritating Alice, who insists that she can fight her own battles. Alice explains her plan for a “Boxcar Library” to Mac, who is deeply impressed. She quickly secures the support of Logan Rutherford, a local mine owner who agrees to provide and outfit a boxcar if the community can supply the books. Her next step is to organize a book drive, which requires the approval of Missoula’s powerful social matron, Julia Walker—Sidney’s mother.

In 1917, a fire in the connected Speculator and Granite Mountain mines, in Butte, Montana, kills 168 men and ignites widespread labor unrest. Colette, now 17, accompanies her father to Butte to join a massive general strike. There, she is deeply moved by a woman reading the final letters of the deceased miners, and her hatred for the Anaconda Copper Mining Company solidifies. While in the crowds, she sees a man being pursued by other workers and deduces from his brand-new, unscuffed outfit that he is a Company spy. She deliberately trips him in order to let the workers catch him. The pursuers’ charismatic, red-haired leader thanks her by kissing her hand before departing. The strike ultimately fails to achieve its goals for the miners. Shortly after their return to Hell Raisin’ Gulch, famed IWW organizer Frank Little is abducted and lynched. Horrified, Colette remembers a line from Shakespeare’s The Merchant of Venice—“If you wrong us, shall we not revenge?” (60).

In 1936, Millie arrives at the dilapidated FWP office in Missoula and meets her new staff. They include Sarah “Flo” Marner, a secretary with dreams of Hollywood; Oscar Dalton, a successful author of pulp noir novels; and Thomas Lyon, a reserved academic and cartographer. The fourth member, Sidney Walker, a photographer, arrives late and is immediately sarcastic and hostile toward Millie and her mission. She confronts the group about their missed deadline and gives a speech about the importance of their work, but she is met with cynicism. Millie realizes that if she is to succeed, she must first earn the trust of this wary and dysfunctional team.

After securing support from mine owner Logan Rutherford—who was wary at first but won over by Alice’s persistence and her family’s standing—Alice approaches Julia Walker for help with a community book drive for her Boxcar Library. Julia is initially polite but becomes alarmed upon learning that the library is intended for adult workers, not just children. At her home, Julia quotes from Upton Sinclair’s The Jungle, arguing that providing such books to miners will fill their heads with “nonsense,” incite strikes, and lead to violence. She fears that an educated workforce will disrupt the profitable but exploitative status quo, calling the camps a “powder keg.” Alice counters that knowledge is not propaganda, but Julia is unmoved and threatens to persuade Mr. Rutherford to withdraw his support. As Alice storms out, she encounters Sidney, who overheard the argument and unexpectedly encourages her not to let his mother stop her.

Millie begins her investigation in Missoula by planning a research trip with her staff. Professor Lyon appears to be her most cooperative colleague. Sidney Walker offers the use of his vehicle and suggests they remove three small towns—Condon, Elmo, and Finley Point—from their itinerary, claiming they were unproductive on a previous trip. Lyon agrees, though he recalls finding a noteworthy bookmobile project in that area. It is tentatively decided that Lyon will stay behind while Millie, Sidney, Oscar Dalton, and Flo go on the trip. When Flo mentions the local librarian, Alice Monroe, Sidney becomes agitated and storms out of the office. On their way to the library, Flo tells Millie that she was the last person to handle the box of essays, having left it on her desk overnight before shipping it. Millie arrives at the library, eager to meet the woman who elicits such a strong reaction from Sidney, her prime suspect.

By 1918, a year after the Butte strike, Colette is deeply involved in union organizing. At a meeting, miners debate arming themselves against the Company. The handsome organizer from Butte, now identified as Finn Benson, a charismatic IWW “Wobblie,” skillfully de-escalates the talk of violence and proposes they start their own newspaper to control their narrative. Later, Colette attempts to recruit a local German boy, Leon Banzhaff, but he angrily refuses, accusing her of moralizing from a position of privilege while his family is struggling to survive. Finn consoles Colette, warning her about professional strikebreakers like the Pinkerton agents. Shortly thereafter, a cave-in occurs at the local mine. Her father returns home with the news that two men died, one of them Leon Banzhaff, who had pushed two others out of the way before the wall collapsed on him.