In Dubious Battle tells the story of young Jim Nolan, who in discovering his purpose in life, aids in initiating a strike by migrant workers against landowners during apple growing season in central California. The novel begins with Jim packing up his few belongings in a paper bag and telling his landlady, Ms. Meer, that he is moving out. Jim heads over to the local Party headquarters (implied to be the Communist Party) where his recruiter, Harry Nilson, conducts an interview and warns Jim about the commitment required of a Party man. Jim is then introduced to the other local members: Dick, Joy and Mac. Dick is described as a young, attractive lady's man with a special knack for brining in supplies and donations. Joy is an older veteran known for his many clashes with the authorities and is somewhat mentally unstable after years of beatings and fights with cops and stints in jail. Mac is the experienced field organizer who spends his time traveling around migrant camps encouraging workers to organize and strike.
Though Jim enthusiastically yearns for action, he is initially given clerical tasks. Mac soon receives a field assignment and is given permission to take Jim along as an understudy. Mac and Jim travel to Torgas Valley, California where the Growers' Association has just announced a pay cut at the beginning of apple picking season. Mac explains to Jim that if the workers get agitated enough to organize and strike, the momentum could spill over into the cotton season, benefiting the workers in the long run. As Mac and Jim enter town and make their way towards a migrant camp, Mac tells Jim they need to find any opportunity possible to gain the trust and acceptance of the workers. Mac soon creates this opportunity after learning that Lisa, the daughter-in-law of one of the men, London, is giving birth without any medical assistance. Mac convinces everyone, including Jim, that he used to work in a hospital and has helped deliver babies before. Mac directs several people in the camp to boil water and gather cloth. Mac successfully delivers Lisa's baby and then later admits to Jim that his actions were improvised and he never actually worked in a hospital nor had any medical training. He reiterates to Jim the importance of taking advantage of any opportunity and using it to advance the cause of the Party.
The next day, Mac and Jim begin picking apples to gauge the workers' opinions about the wage cut and their feelings about a strike. London is visibly upset about the wage cut and after a long day of work, Mac, Jim and London visit Dakin, a friend of London's, who is working at a nearby orchard. London talks to Dakin about getting the men in his camp to organize a strike along with the men in London's camp. Dakin is interested but reluctant.
Soon, rumors about a strike are circulating among the men, but Mac laments that the men need something more to ignite a strike. Jim soon finds himself working in a tree alongside Dan, a seventy-one year old man who boasts that he can work harder than any of the "young punks" in camp. When Dan makes his way down the ladder, the ladder cracks and Dan falls flat on his back. The men see the busted ladder and are outraged at the working conditions in the camp. Mac takes action and gives London and Sam, another worker, quick instructions on how to get the men together to vote for a strike.
Mac then heads to a private orchard owned by Mr. Anderson to arrange a deal. He offers Anderson free labor to pick his apple orchard if he allows the men to live on his land during the strike. At first, Anderson is quite reluctant. However, Mac convinces Anderson that such a deal is in his best interest since Hunter, Gillray and Martin, owners of the Torgas Finance Company and most of the town's assets, still own the mortgage to his property. After Mac argues that the free labor will help Anderson gain his independence by providing him the money he needs to pay off his mortgage, Anderson accepts the deal. On the way back to camp, Mac and Jim are stopped by two men who tell them they know they are "reds" and that they should leave Torgas Valley by morning. Mac and Jim run away from the men and make it back to camp where London has organized a meeting with Dakin and a man named Burke, the strike chairman at a different orchard.
As the men organize and begin the strike, Dick shows up with a medical doctor, Doc Burton, who will ensure that the men camped out on Anderson's property live in sanitary conditions to prevent county health officials from shutting down the camp. Before all the workers from the three orchards fully settle down, five motorcycle policemen park themselves on the side of the road to observe the action, further illustrating the power of the Growers' Association in the Torgas Valley. The next morning, the men gather together and march to town to confront scab workers arriving off of the morning train. Deputies follow them and warn them to leave the scabs alone. When the train arrives, Joy appears in a railcar doorway and moves towards the strikers. The steam from the train masks three shots fired at Joy and he is shot dead.
Mac tells the men that vigilantes shot Joy. He then convinces London to take Joy's dead body back to the Anderson camp to motivate the men. He continues to sense the men's waning interest and commitment to the strike and advises London to send out groups of men to check for scabs working the orchards to give the men something to do. Jim's search party encounters a group of locals protecting the scabs and his arm is grazed by a bullet in the scuffle. He returns to camp in once piece where he learns that Dakin was stopped on the road and ambushed by vigilantes who set his beloved truck on fire. Dakin went mad and when the police arrived, he bit one of the officers and was taken to jail.
Back in the camp, the men organize a funeral for Joy. Mac sees the funeral as an opportunity to unite the workers and reignite their excitement for the strike. Mac convinces London to deliver a speech at Joy's funeral but London is nervous and only musters a few words before turning the eulogy over to Mac. Mac delivers a stirring speech about Joy's life and commitment to the cause. The men then lead a procession to the cemetery to deliver Joy's coffin. After the funeral, the newly elected President of the Growers' Association, Mr. Bolter, shows up at the camp demanding to speak with London. Bolter offers a 5 cent wage increase and nothing else. When London vehemently refuses the offer, Bolter threatens to send out troops to shut down the camp if the strike does not end soon. After Bolter departs, London asks Mac and Jim about their involvement with the Party. The two men tell London they are "reds" and describe their work to London. London tells Mac and Jim that he is not bothered that they belong to the Party. Just then, they see Anderson's barn ablaze. Anderson's entire apple crop, which had been picked and stored in the barn awaiting transport, is burned in the fire.
In the meantime, Mac cannot find the missing Doc Burton. Additionally, Sam, one of the strikers, tells Mac and London that he plans to burn down Hunter's place against their orders. Some of the men round up a high school kid and bring him to the tent were London, Mac and Jim are congregated. The kid admits that he and five accomplices set fire to Anderson's barn but he denies knowing the whereabouts of Doc Burton. Mac roughs the boy up and lets him go. Mac feels bad about punching the boy, but Jim reassures Mac that it had to be done for the cause. They then see smoke in the distance and realize Sam has set fire to Hunter's house. Jim, imbued with a new sense of self and power, tells Mac and London that they need to start making the men follow orders if they are to succeed. Anderson's barn burned because the men sent to protect it abandoned their posts. Doc went missing after his assigned bodyguards let him out of their sight. Jim orders London to police the men and make sure they are following directions. London and Mac are a bit frightened by Jim's vehemence and newfound strength.
The next morning, Mac runs out early to send a letter requesting much needed assistance to Harry Nilson and comes back with a newspaper. The fire at William Hunter's house makes the front page. The report mentions that a special deputy caught the assailant but was assaulted and not expected to live. The paper blames the strikers for the fire at Hunter's house as well as the burning of Anderson's barn. The paper encourages citizens to do what is necessary to drive out the "paid foreign agitators" who are causing the labor problems (229). Mac tells Jim that the strike is probably going to collapse and gives Jim directions to a safe hiding place in case they get separated. Later, the sheriff shows up and tells the men that they have until daylight the following morning to vacate Anderson's land or they will be forced off of the property. Mac tells London they should all stay and fight. He then tells Jim to sneak away back to town at dark. He does not want Jim to risk being caught because he is too valuable to the Party. Mac and Jim are then accosted by a boy who tells them he found a doctor who has been hurt. Mac and Jim start to run, thinking they will find Doc Burton. Realizing they have been duped, Mac stops and tells Jim to duck. Shots are fired. When Mac stands up and calls for Jim, he sees Jim inert on the ground. As he walks closer, Mac sees that Jim's face has been shot off. London and some of the other men soon catch up to Mac. Mac solemnly props up Jim's body for all of the men to see. He begins a speech, similar to the one he gave at Joy's funeral, to rally the men for the cause as the novel comes to a close.