NOTE: This spoiler was submitted by Jeremy

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 written by Bakunin. Soon, Division assassins start making their way toward the building, leading to Elly and Aiden hiding under the floorboards. After a while, Aiden springs out from underneath to start killing the assassins. Before more start coming, the two make their way to the roof and jump off onto a dumpster below.

Aiden brings Elly to his place in London for safety. He goes into the bathroom and talks to his employer with the shower running, and Elly overhears Aiden saying that she “needs a bullet in her head”. Fearing for her life, she grabs Alfie and contacts Ruth so that she can go to Chicago herself (with Alfie). Upon arriving, Elly rejoins Ruth before her father arrives…and it turns out to be RITTER. He starts going through Elly’s stuff before Aiden arrives, telling Elly that these two are not her parents. After Ritter drops the dad act, Ruth (whose real name is Margaret Vogler) pulls a gun (and a British accent) on Elly. Aiden shoots her and knocks Ritter out before getting a horrified Elly out, unfortunately leaving Alfie behind.

Aiden escorts Elly to meet his employer, Alfred “Alfie” Solomon (Samuel L. Jackson). Here, Elly learns “the real Agent Argylle” is none other than…ELLY HERSELF! Her books were not predictions, but rather memories of her past life as a spy. Her real name is Rachel Kylle (Agent R. Kylle), and she lost her memory after a mission gone wrong. Ritter and Vogler got to Rachel before Aiden could, and they posed as her parents, making her believe she was always an aspiring writer so that she can document her thoughts down and eventually bring her down. They were hoping for the latest book to reveal the location of the Master File. Elly is bewildered at the revelation until Aiden engages her in combat, and she is able to utilize her skills from memory to neutralize him.

Solomon tells Elly and Aiden that the Master File is in the possession of a Division associate named Saba Al-Badr/The Keeper (Sofia Boutella). The pair travel to Arabia, where Elly must become Rachel once again. While waiting to speak to Saba at her compound, Aiden starts dancing with Elly, saying that they used to be romantically involved. Saba sees Elly alone and talks to her, where she acts well enough as Rachel for Saba to give her the Master File.

The Division manages to track down Elly and Aiden, capturing them and bringing them onboard their large ship. Elly had just learned that she herself was an asset of The Division. A flashback reveals that she had killed Bakunin before taking the Master File, and the explosion triggered by his apartment self-destructing led to her memory loss. In order to save herself, Elly shoots Aiden in the chest to make Ritter think she is on their side. She also has to pretend that she doesn’t care for Alfie, who is in Ritter’s custody. In reality, she shot him in a specific area so that the bullet would go through his chest but missing the heart, with survival being dependent on stopping the bleeding soon enough, something that Elly got from a fan theory on how Keira could have been brought back. Aiden gets free and kills his captors before going to help Elly.

Elly sees one last hallucination of Argylle, telling her that she never needed him to do what he could do, before she takes on Carlos and his goons, who have to stop with the gunfire after crude oil starts to leak out. Elly then remembers one of her memories being ice-skating, so she straps some blades to her shoes and slides across the oil, slicing through the assassins before fighting Carlos, ending with her throwing a knife into his heart. She shoots the rest of the assassins before she and Aiden take out every last villain on the ship as a couple, with plenty of flair. They make it to the ship’s control room to send the Master File to Solomon, but Ritter aims his rifle at them. Alfie then lunges toward the villain and begins to claw at his face, giving Aiden a chance to shoot and kill Ritter. Unfortunately, Alfie clawed out his eyes, as a retinal scan was needed to complete the upload, but Aiden finds an alternate way.

The duo begin uploading the file to Solomon, but Vogler shows up with a music box that mind-controls Elly to try and kill Aiden, which also makes her put the upload on hold. Aiden fights Elly reluctantly, nearly dying until a masked assailant hops onto the ship and brains Vogler with a wrench, causing her to drop the music box, which shatters upon hitting the floor and severs Elly’s mind control. The assailant is Keira, who survived her gunshot wound the same way Aiden survived his, because it turns out Keira was the one who suggested the theory to Elly. The three (and Alfie) send the rest of the Master File and leave the ship to blow up, killing Vogler and anyone else left alive. Elly and Aiden also finally kiss.

Elly returns to her author role and finishes Book Five, giving Argylle a proper happy ending. She goes back to the book store for another reading, and a fan asks her what happened to the characters after the story. Elly says that, “hypothetically”, Solomon received high honors from the CIA for exposing The Division, while Keira went to rise up in the tech field, and Elly and Aiden appear to be together. Before Elly ends things, another fan who looks much like a nerdier version of Elly’s image of Argylle stands to ask a question, but also suggesting Elly might have questions for him, leaving her surprised.

During the credits, a flashback to 20 years earlier is shown. A young Argylle (Louis Partridge) enters a pub and seems to be working with the Kingsman agency (they exist in the same universe). Text then appears saying “Argylle: Book One”, as a film adaptation of Elly’s first novel is said to be coming soon.

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Elly Conway is an author best known for her "Argylle" series of spy books, starring suave British spy Aubrey Argylle. While trying to come up with a proper ending for her next book, Elly meets a spy named Aiden Wilde, who rescues her from a group of assassins onboard a train. Aiden tells Elly that she is being targeted by a terrorist organization called The Division, led by Director Ritter, as they believe Elly's books predict events in their line of work, and they need her to finish her next book to track down a Master File that would expose all of The Division's operatives.

After eluding other assassins, Elly runs away with her cat Alfie to her parents home in Chicago. It seems as though Ritter is Elly's father along with her mother Ruth, but Aiden lets her know they are both posing as her parents, and Ruth is actually an evil British scientist named Margaret Vogler. Aiden then brings Elly to his employer, Alfred "Alfie" Solomon, where Elly learns that "the real Agent Argylle" is actually herself, as she was a former spy named Rachel Kylle (Agent R. Kylle) who lost her memories after a mission gone wrong. Ritter and Vogler got to her first and pretended to be her parents so she can write the books and give up her secrets.

Elly and Aiden retrieve the Master File from a woman named Saba, but The Division captures them. The duo fight off the assassins before killing Ritter and trying to upload the file to Solomon. Vogler tries to mind-control Elly using a music box to kill Aiden, but one of their former associates, Keira, shows up to whack Vogler over the head and break the music box. Elly is freed, and they finish uploading the file before they destroy the ship and flee.

Elly returns to writing and finishes the Argylle series with a happy ending for the famed spy and his friends. Elly is then startled to see a man that looks much like how she envisioned Argylle to be asking her a question.