The film is a spin-off set within the highly secretive and lethal world of the John Wick universe, focusing on a young assassin named Rooney (Ana de Armas). Rooney was briefly introduced in John Wick: Chapter 3 – Parabellum as a deadly dancer trained in the same Ruska Roma school where John Wick once trained.

The story begins after Rooney has left the formalized assassin life and is attempting to lead a quiet existence. However, her past is brutally dragged back into the present when her entire family is murdered in a targeted hit, which she initially believes to be a random tragedy.

Driven by revenge, Rooney uses her extensive combat and infiltration skills to begin an investigation. She discovers the massacre was ordered by a powerful, high-ranking figure within the criminal underworld, likely connected to the High Table or a rival syndicate with whom the Ruska Roma clan had a historic debt or conflict.

Rooney must re-engage with her old life, utilizing her training as a ballerina where precision and grace mask lethal intent to track her targets. Her journey is fraught with danger and leads her across Europe, from the opulent opera houses that serve as clandestine meeting points to the gritty underbelly of various organized crime rings.

She seeks counsel, and eventually reluctant assistance, from key figures in the Wick universe, including Winston Scott (Ian McShane) and The Continental Manager, who provide her with intel, weapons, and necessary logistical support, but warn her of the escalating consequences of defying the High Table's rules.

The climax involves Rooney executing a complex and highly choreographed assassination attempt during a grand public event (possibly an opera performance or major gala). Her target is the syndicate boss responsible for her family's death. The final battle is a spectacular sequence combining balletic movement with brutal close-quarters combat, utilizing the environment and her training to devastating effect.

Rooney achieves her bloody revenge, but in doing so, she fully sacrifices her chance at a normal life, further entangling herself in the high-stakes world of assassins. The film concludes with Rooney walking away from the scene, having made a decisive choice to embrace her life as a lethal operative, leaving her future trajectory in the criminal underworld uncertain but cemented.