The film kicks off in Greece, where the charming British spy Aubrey Argylle is gearing up to confront his target, the dangerous terrorist LaGrange. Their initial interaction involves a captivating dance, but it quickly turns tense as LaGrange commands her henchmen to aim their weapons at Argylle. In a race against time, he reaches out to his tech-savvy partner Keira, who works diligently to devise an escape route for him. Just when it seems like Argylle might evade disaster, LaGrange catches up, firing a shot that grievously injures Keira. Desperate to save her, Argylle calls upon his ally Wyatt to capture her before she can make her escape. After a tense confrontation, Wyatt apprehends LaGrange, leading both men to interrogate her about her employer, who turns out to be Fowler (Richard E. Grant), the director who oversees Argylle and Wyatt’s missions. In a shocking twist, LaGrange poisons herself, leaving Argylle to question who he can trust.

The narrative shifts to a book reading by Elly Conway, the author of the beloved “Argylle” book series, where the previous scene is revealed to be the climax of her latest installment. After a successful event, Elly retreats to her secluded cabin with her loyal feline companion, Alfie, to immerse herself in writing the fifth book. That night, she believes she has completed her draft only to be critiqued by her mother, Ruth (Catherine O’Hara), the next morning. Ruth finds the story compelling but has reservations about the ending, calling it a “copout.” This propels Elly into a quest to navigate her writer’s block, envisioning various potential endings for Argylle while struggling to find inspiration.

Elly embarks on a train journey to visit Ruth in Chicago, hoping for guidance on her new chapter. Onboard, she encounters Aiden Wilde, a man who initially poses as a fan but warns her of imminent danger from would-be assailants targeting her. Just as a “fan” approaches with a concealed knife, Aiden springs into action to protect Elly, who, in her mind, sees Argylle taking on the role of her brave defender. Aiden’s heroics save Elly, and together they flee the chaotic train via parachute, with Alfie in tow.

Waking in a safe cabin, Aiden reveals that the assassins belong to a nefarious group known as The Division, led by the chilling Director Ritter. The Division believes that Elly’s stories foreshadow real-life events and are fixated on her unpublished manuscript for Book Five. In a chilling moment, Ritter is seen executing his Deputy Director (Rob Delaney) for failing to secure Elly and a crucial Master File that could expose all Division agents. The twists deepen when it is revealed that Wyatt was inspired by Aiden, while Keira was a real casualty that Aiden mourns.

The duo attempts to track down a vital informant named Bakunin, as Elly slowly begins to realize the power her narratives hold. At a suspicious apartment connected to Bakunin, they find only a hidden compartment with a layer of chaos looming overhead. As Division assassins close in, Aiden fights valiantly while Elly, hidden beneath the floorboards, channels her character Argylle in a moment of crisis. They narrowly escape, ultimately finding refuge in London where more revelations await.

As they unravel the mystery, secrets of Elly’s true identity come to light. To her shock, she learns that she is not merely an author but the real-life counterpart of Agent Argylle, whose memories were buried after a disastrous mission. The facade that Ritter and another accomplice, Margaret Vogler, constructed around her life crumbles, revealing a web of deceit that has plagued her existence.

Elly and Aiden embark on a perilous journey to recover the Master File from another operative, Saba Al-Badr/The Keeper. The stakes escalate dramatically as they find themselves pitted against The Division’s forces once again. Elly must embrace her true identity as Rachel Kylle and revert to her former spy skills to survive. Their mission leads them on a thrilling chase, connecting past relationships and igniting a spark between Aiden and Elly, who rediscover their chemistry amidst the chaos.

As they navigate the treacherous paths laid by their enemies, a final confrontation ensues on a Division ship, where choices made under pressure determine their fates. Elly’s ingenuity shines through as she navigates her complex feelings for Aiden and her responsibility toward Alfie. The climax unfolds as they fight against overwhelming odds, reclaiming control over their lives and destinies.

The film concludes on a poignant note, as Elly returns to her writing, crafting Book Five with the resolution she had long sought for Argylle. In a narrative epilogue, she teases further adventures in her series, deeply intertwining her reality with her fictional creations. As the credits roll, a nostalgic flashback reveals the younger Argylle, hinting at a larger universe at play, as fans eagerly await the adaptation of Elly’s first novel, setting the stage for future exploits.