In 1971, the executives at McGraw-Hill express interest in Clifford Irving's novel Rudnick's Problem, following the lackluster performance of his previous work, Fake!, about art forger Elmyr de Hory. Though Irving believes he has crafted a potential bestseller, the publisher decides against its release after a Life editor finds it unsatisfactory.
During a vacation with his friend and researcher, Richard Suskind, Irving faces an unexpected turn of events when the eccentric billionaire Howard Hughes demands that everyone vacate their hotel. Forced to return to New York City, Irving discovers that he is now relegated to meeting with an assistant rather than any decision-makers. Frustrated, he storms into the boardroom, proclaiming that he has a project which will be the "book of the century," and threatening to take his talents elsewhere unless they're taken seriously. In search of a worthy subject, he stumbles upon a cover story on Hughes and quickly decides to pursue him as the focus of his new work.
Irving fabricates a story that Hughes has engaged him to write his autobiography and presents forged handwritten notes purporting to be from Hughes as evidence. To his surprise, handwriting experts authenticate the notes, leading McGraw-Hill to agree to a $500,000 deal for the book. Irving operates under the assumption that Hughes, known for his reclusiveness, is unlikely to contest the publication, and that his eccentric behaviors can help him navigate any challenges to the book's authenticity.
As Irving's personal life becomes strained due to marital issues with his artist wife, Edith, arising from an affair with Nina van Pallandt, he reassures her of his faithfulness as he embarks on research trips with Suskind. They pour over documents related to Hughes, even illegally acquiring a draft biography from a former aide, amassing information to strengthen their claims. Irving adopts Hughes's persona, mimicking his style and even recording passages in character.
As they delve deeper, Irving receives a mysterious box containing scandalous details regarding dubious dealings between Hughes and President Richard Nixon. Convinced that Hughes instigated this delivery to endorse his work, Irving integrates this sensitive information into the manuscript.
As the release date looms, Irving escalates his ruse by staging a phony meeting between Hughes and the publishers. Despite Hughes’s public denials, the McGraw-Hill executives remain convinced of the book's authenticity and optimistic about its successes. Irving negotiates larger payments for himself and a purported Hughes while he and Edith scheme for her to funnel a check from Hughes, made out to "H. R. Hughes," into a Swiss bank account under a forged identity, "Helga R. Hughes."
The mounting pressure transforms Irving's reality into paranoia as he grapples with guilt over his ongoing affair and the strain of maintaining appearances with Edith. Following a televised conference call, during which Hughes publicly denies any knowledge of the book, Irving spirals further into desperation. He finds himself arrested and, in a bid for cooperation, insists on immunity for Edith. Ultimately, they all receive minor sentences.
A radio broadcast hints at a flurry of favorable legal rulings for Hughes, which leads Irving to surmise that his book has embroiled the president in a precarious situation financially. The narrative alludes to the turbulence within Nixon’s administration, suggesting that his fixation on Hughes may have played a role in the notorious Watergate scandal, a point debate among historians and political analysts.
