Plot summary

The film opens in Greece, where suave British spy Aubrey Argylle (Henry Cavill) prepares to move in on his target, a terrorist named LaGrange (Dua Lipa). After a brief dance together, LaGrange has her assassins draw their guns on Argylle. He then contacts his partner Keira (Ariana DeBose), who uses her tech to find a way for Argylle to get out. LaGrange then corners Argylle and Keira outside and fires at them, hitting Keira in the chest. As Argylle tries to save her, he contacts his other associate, Wyatt (John Cena), to catch LaGrange before she can escape. Wyatt catches her, and he and Argylle interrogate LaGrange to learn who her employer is. It turns out she is working for Fowler (Richard E. Grant), the director who also employed Argylle and Wyatt. LaGrange poisons herself before she can be brought in, and Argylle now wonders who he can really trust.The scene shifts to a book reading by Elly Conway (Bryce Dallas Howard), author of the "Argylle" book series that the character originates from, as the entire scene was from the last chapter of the fourth book in the series. After answering some questions, she later goes to her private cabin with her pet cat Alfie so that she can begin to write the fifth book. She spends the evening writing the whole thing, thinking she has completed it before going to bed.In the morning, Elly is contacted by her mother Ruth (Catherine O'Hara), who spent all night reading the manuscript Elly sent her. Ruth says the book is good, but she doesn't like the ending, calling it a "copout". Elly then starts trying to write another new chapter to try and give the book better closure. She envisions several scenarios to give Argylle a better ending, but she finds herself drawing a blank.Elly decides to take a train with Alfie to visit Ruth in Chicago when she suggests to help Elly finish the book. On the train, she is approached by a man named Aiden Wilde (Sam Rockwell). He pretends to be a fan of Elly's before informing her there are people on the train who are targeting her. Moments later, a "fan" comes by with a knife disguised as a pen, forcing Aiden to jump into action and start fighting assassins while Elly tries to hide, though she visualizes Argylle himself in Aiden's place. One of the assassins is a man named Carlos (Tomas Paredes), whom Elly recognizes from the book reading as a supposed fan. Aiden then takes Elly to the end of the train where he blasts the doors off and parachutes with her (and Alfie) out of the train, where she passes out midair.When Elly wakes up, Aiden has cut his hair and reveals he took her to a cabin. He informs her that the assassins come from a sinister organization called The Division, led by Director Ritter (Bryan Cranston). They believe that Elly's books predict certain events in their line of work, and they have already read her manuscript for Book Five and are waiting on her to write the final chapter. Ritter is shown executing his Deputy Director (Rob Delaney) for failing to apprehend Elly and reacquiring a Master File that can expose all agents of The Division. Furthermore, the character of Wyatt is based off of Aiden, while Keira was a real person who apparently really did die.Aiden brings Elly (and Alfie) along with him to find an informant named Bakunin (Stanley Morgan), who was going to deliver the Master File to him. Hoping to get an idea of where to find Bakunin, Aiden convinces Elly to start writing the next chapter, which Ritter and his men listen in on because some fans record Elly on their phones.The duo are led to an apartment supposedly belonging to Bakunin. They find nothing but an opening under the floorboards, where Aiden finds a book with information