NOTE: This spoiler was submitted by Jeremy

1940 – A young Orson Welles (Tom Burke) is tasked by the struggling RKO Pictures to create a film, where he is granted total control over the product. He is allowed to work with anyone he wants, and he chooses to collaborate with the writer Herman J Mankiewicz, AKA “Mank” (Gary Oldman).

At the North Verde Ranch in Victorville, CA, Mank, suffering from a broken leg, is brought in to take 90 days to write his screenplay for Welles’s film. Helping him is secretary Rita Alexander (Lily Collins) and housekeeper Fraulein Freida (Monika Gossmann). Producer John Houseman (Sam Troughton) instructs Mank on what to do per Welles’s expectations. Also knowing that Mank is an alcoholic, Houseman provides a case with bottles of nightcaps. Welles then calls the house and speaks to Mank, saying he wants the script done in 60 days now. Throughout his stay there, Rita logs down Mank’s notes and writing down as he lays bedridden.

Three weeks earlier at the Mankiewicz house, Mank’s wife Sara, or “Poor Sara” (Tuppence Middleton), is helping him, barely able to stand on his feet, into bed as he rants about how “The Wizard of Oz” is going to sink its studio. Shortly after, Mank is in a car ride with a young man, Tommy (Sean Persaud), who gets them into an accident after trying to show Mank a letter from a woman. This is where Mank gets his broken leg. Welles then visits him in the hospital to begin his proposition on their collaboration.

Flashback to 1930 at Paramount Studios, where young screenwriter Charles Lederer (Joseph Cross) arrives at Mank’s behest. Charles meets Mank’s creative team after they engage in a brief bet. There is George Kaufman (Adam Shapiro), Sid Perelman (Jack Romano), Charles MacArthur (John Churchill), Shelly Metcalf (Jamie McShane), and Mank’s brother Joseph (Tom Pelphrey). The men gather to pitch their film idea to David O Selznick (Toby Leonard Moore), which sounds vaguely like “Frankenstein”. However, Selznick and director Josef Von Sternberg (Paul Fox) don’t seem interetsed. After the pitch, Charles invites Mank to visit his aunt.

Mank agrees to the invtiation and shows up to a film set, falling down drunk. After a nap, Mank leaves his room and goes to check out the set, where he sees Metro-Goldwyn-Mayer producer Irving Thalberg (Ferdinand Kingsley) and the studio’s chairman, Louis B. Mayer (Arliss Howard). Irving recognizes Mank from writing a few of their Lon Chaney films. They are overseeing the production, which is starring actress Marion Davies (Amanda Seyfried), Charles’s aunt who mutually recognizes Mank from a previous encounter. After chatting with her, Mank meets William Randolph Hearst (Charles Dance), who is fronting the production and is in an affair with Marion (who calls him “Pops”). Hearst sees a bright future in Hollywood despite coming from the business of journalism.

Present – Mank speaks to Sara on the phone while Rita reads a message she believes is from her husband serving in the British Army. It is reported that his ship was sunk off the coast of Norway, and he is presumed missing. Rita leaves the house in despair. After a while, she returns to find Mank passed out drunk from taking one of the nightcaps from the case. She calls Houseman and a doctor to check on him. When Mank wakes up, Houseman tells him that the story he has written so far doesn’t make much sense. While discussing the story more with Rita, she learns that some characters are based on people Mank knows. Joe even later calls Mank to express concerns over how he is writing the script and who it may piss off, but Mank doesn’t care.

1934, MGM Studios – Mank and Joe meet with Mayer as he is throwing out a man pitching a smut picture. The two are beginning to work for him as Mayer expresses his passion for films and his studio. They follow him as he speaks before a live audience to ask for donations for the studio, or else he will have to roll back salaries of his employees. After deliving a speech, he gains favor from the audience, although Mank considers the whole act to be in bad taste.

A little over a year earlier, Mank and Sara are gathered at the San Simeon Assembly Room at Hearst Castle for Mayer’s birthday. The attendees discuss politics, that of the current Nazi Germany, as well as the writer Upton Sinclair (Bill Nye – yes, the science guy), whom Mayer and Irving consider to be a socialist, but Mank appears to show support for the man. Marion steps outside, and Mank follows. The two walk around the estate and continue discussing politics, as well as Marion’s hometown of Brooklyn. Mank also takes the time to criticize Mayer.

Present – Houseman goes to the ranch to remind Mank that he has barely two weeks left to complete the script. Welles calls to check on the progress since he has finished filming “Heart of Darkness”, and Mank makes excuses. Rita finds out that Frieda has been giving Mank the nightcaps that he has been consuming almost every night. When Rita asks Frieda why she does this, Frieda tells her that Mank was responsible for getting her and her family safely and legally out of Hitler’s Germany.

Back in 1934, Sinclair is running against Frank Merriam for governor of California. Joe also tries to get Mank to get involved with the establishment of the Screen Writers Guild, but Mank is not interested. He is then called in to meet Irving, since MGM is backing Merriam, and Mank hasn’t been donating to the funds for him. On his way out, Mank sees a rally with Sinclair speaking, clearly against MGM as well.

Present – Mank has managed to come up with 200 pages in 13 days after receiving a support device. Houseman praises the script, even commenting that he never knew someone could care so much about a sled. However, Mank is told that because he signed a contract, he cannot receive credit for the film. Houseman also knows that the protagonist of Mank’s script is based off of Hearst.

1934 – Mank and Sara go to the beach, where they hear a radio ad in support of Merriam. The couple recognizes the voice of a well-known actress who is not a “poor widow” as she is making herself out to be. At MGM, Mank and his team must work on phony smear ads against Sinclair, which he is not happy to be doing. Mank finds Marion and tries to get her to tell Mayer that Hearst wants the Sinclair ads pulled, but she knows she wouldn’t be able to convince him.

Sometime later at the Trocadero Nightclub, everyone gathers on the eve of the election as they await to hear the results. After a few hours, Mayer announces Merriam as the new governor, receiving thunderous applause. Mank is then called by Shelly, who is drunk at the studio and lamenting Sinclair’s loss. After Shelly fails to tell Mank where he is, he calls Shelly’s girlfriend Eve (Leven Rambin), who tells him that Shelly has a pistol with him. Mank eventually finds Shelly at the office, where he tells Mank that he has been diagnosed with Parkinson’s. Mank orders Shelly to hand over the gun, but he instead gives him the bullets. When Mank returns to Eve, she tells him that Shelly had a whole box of them. Cut back to the office where a flash is seen and a gunshot is heard.

Present – Mank is visited a couple of times by Joe to discuss the script, now even more concerned with how it is written, even stating that the “Rosebud” in the film is seen as a thinly-veiled reference to Hearst’s nickname for Marion’s vagina. Marion herself later visits Mank and asks that he change the script for Hearst’s sake, but Mank refuses. She apologizes in advance if the film does not get made because of her. Welles later calls Mank to tell him that they are having trouble with getting the film made since Mayer tried to bribe RKO into shelving the film for Hearst.

In 1936, Mank attends Irving’s funeral. He runs into an old colleague who offers him a job at another studio, but Mank states that he already received this invitation before and was denied a meeting. In 1937, Mank goes to a party at Hearst Castle, spitting drunk. He starts to pitch his film to the whole crowd, essentially using it as a means to spitefully speak against Hearst, Mayer, and even Marion before he vomits and sends everyone out of the room. Mayer rips into Mank for his behavior, but Hearst speaks to Mank calmly. He tells him an allegory of an organ grinder and his monkey, comparing him to the organ grinder who depends on the monkey to dance and earn payment before he sends Mank out.

Present – Sara sees Mank to call him out on his behavior and dishonesty, stating that he owes her, as well as saying that Joe mentioned Mayer can give him his old job back if he apologizes to him. She also tells him that she doesn’t want anyone calling her “Poor Sara” anymore. Welles later visits Mank to offer him a buyout from the studio. Mank tells Welles that he wants credit on the film since he knows it’s the best thing he’s ever written. Welles is furious and even smashes Mank’s case of nightcaps, but Mank instead sees that as a good thing to add to the script. After Welles leaves, Rita receives another message stating that her husband is confirmed to be alive, and she hugs Mank.

1942 – At the Academy Awards, the film, “Citizen Kane”, wins its only award, for Best Original Screenplay. Mank and Welles both win, but neither show up to collect the award. A radio broadcast with the real Orson Welles is heard, where he sends a message to Mank to “kiss my half”. Mank is shown accepting the award in front of his house, saying that he had no help at all from Welles, and when asked by a reporter why Welles has co-credit, Mank says, “That’s the magic of the movies.”

The final text states that Herman Mankiewicz died eleven years later from alcohol complications at the age of 55, and he had never worked on a major film production ever again. He had stated to a friend that in his final years, he had begun to feel trapped like a rat, always sabotaging himself when he found an escape.

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In 1940, alcoholic screenwriter Herman J Mankiewicz/"Mank" is tasked by Orson Welles to write a screenplay for RKO Studios. Flashbacks are shown of Mank's time working at Paramount Studios before moving to MGM to work under chairman Louis B Mayer, who is backed by businessman William Randolph Hearst.

Mank disagrees with Mayer and Hearst's political views, and he and his team are forced to create propaganda videos against writer Upton Sinclair, who is running against Frank Merriam for governor of California. Mank also develops a friendship with Hearst's mistress, the actress Marion Davies. The film jumps between the past and present, showing that the characters in Mank's script are based on people like Marion and Hearst. His secretary Rita and brother Joe are among those who take notice, while Joe, Marion, and producer John Houseman urge Mank to make changes, or else he will anger Hearst.

Mank burns bridges with both Mayer and Hearst at a party at Hearst's estate when he drunkenly pitches his film to everyone in attendance, which is also a stealth attack against the two men and Marion. Years later in the present, Mank has finished his script, which is widely praised, but when he tells Welles he wants credit (after previously signing an agreement to not get any), the two end their professional relationship.

The film becomes "Citizen Kane", and both Mank and Welles win the Oscar for Best Original Screenplay. End text states that Herman Mankiewicz never worked on another film again and died eleven years later at the age of 55 from complications to alcoholism.