The movie starts in the year 1947 with Dalton Trumbo (Bryan Cranston) in his bathtub, writing a script on a typewriter. At this point in time, he is an established screenwriter in Hollywood. We next see Trumbo on set as the film he wrote is shot. He attends a party with Edward G. Robinson (Michael Stuhlbarg). Trumbo goes to the movies and watches a film reel where gossip columnist, Hedda Hopper (Helen Mirren), speaks of many in Hollywood being Communists, showing Trumbo as an example; the House Un-American Activities Committee has been formed to investigate. After the film is over, a patron recognizes Trumbo and throws his soda at him.
Back at his ranch, Dalton's young daughter Niki asks him if he's a Communist. He tells her it's not against the law and he simply wants a better government. He gives her the analogy if she saw someone at school who didn't have a lunch, would she share hers or tell the student to get a job? Dalton and others, including Arlen Hird (Louis C.K.) meet with Edward G. Robinson at his mansion; they discuss the scare tactics Congress is stirring up. At a meeting for the Motion Picture Alliance for the Preservation of American Ideals, John Wayne gives a speech vilifying the Commies and declaring that they need to answer some questions. Arlen and Trumbo hand out pamphlets on free speech but John Wayne argues with them and ostentatiously tears one up while Hedda Hopper looks on. Trumbo points out that he served in the military but John Wayne hasn't - the closest experience he had was on a film set, wearing makeup. Hedda Hopper hints that she will put the exchange in her next column.
Trumbo goes to MGM Studios and meets with Louis B. Mayer who tells him that his next deal is going to make him the highest paid writer in Hollywood. But Mayer points out Trumbo has been featured in Hedda Hopper's new column and says he doesn't want to see an article like that again. Trumbo suggests he stop reading Hedda Hopper.
Trumbo and his friends and family gather at his ranch. They mention Trumbo's record-breaking, three-year contract with MGM. Trumbo's wife, Cleo (Diane Lane), demonstrates her juggling skills at her daughter's urging. We learn she was an acrobat as a child. The party is interrupted by a man who serves a subpoena to Trumbo, suggesting he and others in Hollywood are using the movies to corrupt democracy and overthrow the nation. We see some news reels of the hearings, then hear Trumbo and the others deciding to answer questions ambiguously. Some worry this will be contempt of court but someone else points out that they can appeal and the Supreme Court will side with them there's a five to four liberal majority who will agree the Committee is unconditional. In court, Trumbo is one of ten forced to testify along with Arlen, who is hesitant and can't afford the legal fees. Trumbo offers to cover him.
In the U.S. Capitol, the hearings take place. Trumbo is defiant of the questioning, pointing out he hasn't been accused of a crime. Arlen follows suit, responding to questions with humor. Outside, Arlen admits to Trumbo he has recently found out he has lung cancer. The Hollywood 10 that testified are charged with contempt of Congress. At MGM, Hedda Hopper meets with Louis B. Mayer. She threatens to name him as a supporter if he doesn't fire Trumbo and the others, reminding him of her days as an actress when he tried to force her to have sex with him. Cut to Louis B. Mayer announcing he is dissolving ties to any of the Hollywood 10 under contract at MGM.
At Edward G. Robinson's mansion, he collects money for a defense fund for the Hollywood 10. Trumbo realizes Robinson has sold one of his paintings to help fund the cause. The group is found guilty of contempt of Congress. Trumbo's lawyer tells the press the Supreme Court will side with his defendant. Privately, Trumbo admits he is now broke since his three-year contract was annulled the defense is going to cost him $90,000. Trumbo gets to work writing a new screenplay. Buddy Ross, a Hollywood producer, agrees to produce the films Trumbo is writing, independently, as soon as the court case finishes. Trumbo asks him if he would ever name names if he was on trial for being a Democrat Buddy says he wouldn't.
Trumbo continues to write but just can't put his own name on his work. He instead gives the credit to Ian McLellan Hunter (Alan Tudyk) and asks him to sell it to the studio in exchange for 30 percent of the payout. The script he's written is Roman Holiday. Meanwhile, contractors are working on the ranch; one mentions to Cleo that they haven't been paid. Right then, Trumbo enters, declaring that he and Cleo are rich - Roman Holiday just sold to Paramount. Cleo is distracted though because one of the liberal judges on the Supreme Court has died, right after another liberal Supreme Court judge. There now is no longer a liberal majority and their appeal is going to be denied, securing the prison sentence that they were given.
Trumbo reports to prison for his one-year sentence. He does his best to get along with other prisoners but feels very out of place. Outside, Edward G. Robinson is being questioned at the U.S. Capitol about whether or not he had political meetings attended by Communists at his home. He is asked to name names and Edward does, naming many including Arlen Hird and Dalton Trumbo. Unknown to him, Hopper contemptuously still wants Edward kept on the blacklist regardless of his cooperation, but John Wayne is outraged at this potential double cross for someone he felt did the right thing and successfully demands Hopper to now leave Edward alone.
Time goes by. Trumbo feels some measure of satisfaction as he finds J. Parnell Thomas (James DuMont), the senator who interrogated him so harshly at the congressional hearings, a fellow prisoner after being convicted of tax evasion. We see news footage of the Rosenberg espionage trial, where a couple is found guilty of selling atomic secrets to Russia. And of Joseph McCarthy focusing on uncovering Communists in the United States. Trumbo is released from prison and reunites with his kids, who are now much older (Niki is now played by Elle Fanning). His house has been sold. Trumbo runs into Buddy Ross at a restaurant but Buddy blows him off.
Arlen suggests they sue the studios but Trumbo suggests they instead make money by continuing to write screenplays. He visits Frank King (John Goodman) at King Brother Pictures, which creates B-movies. Frank says they can't afford him but Trumbo agrees to write screenplays at the same rate as other writers they've hired and will have it done in three days. Trumbo locks himself in the bathtub and bangs out a script. When the script is delivered, Frank loves it. He pays Trumbo and then asks Trumbo to fix other scripts they've had. Trumbo and his family move into a new home. A neighbor has written a note, calling him a traitor, and vandalizing his pool.
Roman Holiday is a huge hit. Trumbo continues banging out scripts for the King Brothers who want quantity over quality. He's overwhelmed by the demand so he gets Frank to hire Arlen Hird and Ian McLellan Hunter, who is now blacklisted despite getting an Oscar nomination for Roman Holiday (which Trumbo actually wrote). Trumbo asks his family for their help in keeping the business going they will answer the phone and if they are asked for one of Trumbo's pseudonyms, he will take the call. They'll hand over scripts to messengers at their door based on what name they're under. And Cleo and Niki will make deliveries. Niki is concerned because she's consumed with schoolwork and civil rights projects.
Frank King has issues with Arlen's script but assigns Dalton to fix it, despite him not getting paid for rewrites. Arlen is embarrassed to be writing crappy screenplays and keeps integrating political themes into his works. He points out that he was nominated for a Pulitzer and Trumbo won a National Book Award. Trumbo tells Arlen about an idea he has for a screenplay about a bullfighter. Later, the Oscars telecast is on and Roman Holiday wins Best Screenplay. Buddy Ross calls Trumbo at home and asks if it's true that he had actually written Roman Holiday he needs a script and wants to hire Trumbo. Arlen and Trumbo argue, with Trumbo saying if they continue to work despite being blacklisted, they win. Arlen saying he wants to make changes, not just get money and accolades.
Jumping forward in time, it's now Niki's 16th birthday. Trumbo is locked in the bathroom, working on a script. Niki tries to get him to come downstairs for cake and he yells at her for interrupting him while he is working. She runs off in tears. Cleo comforts her as she complains about her father.
Dalton finds out Arlen has finally died from the cancer. Trumbo attends the funeral, then visits Edward G. Robinson to tell him the news. His walls are now adorned with paintings. Trumbo wants to repay the money Edward gave them for their defense fund, so they're not indebted to him (after he named their names). Edward explains he just wanted his life back. Dalton tells him he didn't have to give names since Congress had no right to demand it of him. Edward tells him he probably shaved years off Arlen's life by forcing him to commit contempt against Congress. In a restaurant, Hedda Hopper approaches Trumbo. She asks him to solidify the rumor that he's been writing popular screenplays under false names. He refuses to give her an exclusive. Hedda reveals that Buddy Ross named names behind closed doors, including Trumbo's.
Trumbo asks his son to deliver a script to a film set in Agoura, 50 miles away. Niki points out Chris has a date that night. Trumbo suggests Niki does it instead but she is going to be protesting racial segregation. He is adamant she deliver the script first; she refuses and rushes off. Trumbo then forces Chris to abandon his plans to go to the movies. Niki doesn't come out that night.
Cleo tells Trumbo that when she was younger, she chose Trumbo over another boyfriend because that other boyfriend would have been an abusive father. She tells Trumbo he's losing them, always barking orders after returning from prison instead of talking to them like he used to. He is defensive and she tells him she won't let anybody bully their children. Trumbo finds Niki at a protest. He apologizes for being so aggressive but lately he's wired just to fight. Niki says she always wanted to be just like her father; Trumbo says she is. She forgives him.
The IASTE Union Leader, Roy Brewer, visits Frank King and tells him he knows hes employing Trumbo and other Communists. He threatens to expose the King Brothers, which would result in protests and boycotts. Frank responds by picking up a baseball bat and smashing things, telling Roy he isn't afraid of boycotts; all of his movies are garbage. He says he is in film for the money and the pussy and if Roy takes that away from him, he won't sue him but he'll beat him to death with the bat. Roy runs off. Trumbo arrives with a new script, The Brave One, using his bullfighter idea. He explains to Frank that it's too good to be made on the cheap.
Months later, the film comes out and the family goes to see it although it's credited as being written by Robert Rich. Later, the screenplay is nominated for an Academy Award. But unlike Roman Holiday, which was accredited to his friend, there is no Robert Rich to get the credit if it wins. And it does. Hedda Hopper storms the Motion Picture Alliance Offices and demands Roy Brewer tells her who Robert Rich is, hoping it's not who she thinks it is. Journalists interview Trumbo, asking if he's Robert Rich. He is ambiguous in response. Kirk Douglas comes to visit, asking if Trumbo can work on the script of his new movie. He does, it's called Spartacus and credited to Sam Jackson. Hedda meets with Kirk and threatens to send him to court if he hired Dalton Trumbo. Kirk says he doesn't like the way things are and isn't scared of Hedda.
Otto Preminger, the director, has read Trumbo's Spartacus script and wants to hire him to write Exodus for Paul Newman to star in. He waits impatiently during Christmas morning, forcing Trumbo to write pages of the script. He doesn't like the work and says if he keeps it up, he'll put Trumbo's real name on the script to shame him. More new pages and Otto demands the work be more genius. Trumbo says if every scene is is brilliant, the work will be monotonous. Otto tells him to write every scene brilliant and he'll direct unevenly. Kirk visits and asks for some new revisions on the Spartacus script. Trumbo can't begin working on it again for another week. He tells Kirk that Otto said that if he keeps up that level of work, he'll see to it that his name is on the movie twisting his words to make it seem like Otto might give him a screen credit. He then tells Otto that Kirk came by and they discussed him getting a screen credit on Spartacus.
On the set of Spartacus, the head of Universal tells Kirk that Hedda Hopper is predicting a boycott unless they fire Trumbo. Meanwhile, the King Brothers have several lawsuits against them from writers claiming that they are Robert Rich and wrote The Brave One. In order to settle the cases, they have to reveal Trumbo was the real writer. Niki points out that the neighbor that had called Trumbo a traitor upon moving in has seen Kirk Douglas and Otto Preminger visit and has already figured out Trumbo is writing all of these scripts with mysterious writers. Trumbo will legally be tied to the film and will receive the Oscar. Trumbo admits that he is Robert Rich to a news reporter. He uses this as an opportunity to point that the House Un-American Activities Committee has yet to expose any Communist conspiracies or write any laws yet invested millions of dollars in their investigation. The only thing they've accomplished is keeping people from working. Yet he still managed to win two Academy Awards. Trumbo is able to use this interview to turn public opinion against the blacklist. In response, Otto announces his new film is written by Dalton Trumbo. Kirk also demands that the head of Universal announce Trumbo wrote their screenplay, as well. When he refuses, Kirk threatens to leave the film despite already having shot half the movie (meaning those millions of dollars would have been wasted).
Right before its release, Hedda threatens a mass protest of Spartacus unless Trumbo's name is removed. When the Universal head refuses, she threatens it will be the end of his studio. Nonetheless, the film premieres with Trumbo's name in the credits. Cleo begins to cry, stating that it's all over now (i.e., the blacklisting). President John F. Kennedy is asked about his thoughts on Spartacus and the controversy. He responds that he thinks it's going to be a big hit, indirectly supporting Trumbo and denouncing the blacklist. Hedda Hopper, at home, glares at the television, humiliated at her defeat.
In March 1970, Dalton Trumbo is given the Writers' Guild of America's Laurel Award. In his speech, he notes that the blacklist was a time of evil but people shouldn't think of the people involved as heroes or villains but every single one of them, victims.
Title cards say that Trumbo received his Oscar for The Brave one in 1975, and died a year later at age 70. Cleo accepted an Oscar awarded posthumously for Roman Holiday in 1993, and she died in 2009. Hundreds of Hollywood names were blacklisted, and thousands more unfairly targeted. Teachers, soldiers, government workers and their families suffered terrible losses including suicides. The House Un-American Activities committee continued their investigations until 1975.