"You're Under Arrest: No Mercy!" is a two-episode OVA that serves as a continuation of the popular "You're Under Arrest" franchise. The story is set in the fictional Bokuto Police Station and revolves around the daily lives and duties of its Traffic Division officers, particularly the duo of Natsumi Tsujimoto and Miyuki Kobayakawa. The narrative combines action, comedy, and drama, maintaining the series' signature blend of lighthearted moments and intense police work.

The plot begins with the Bokuto Police Station facing a crisis when a mysterious hacker infiltrates their systems, causing widespread chaos. The hacker, known only as "Phantom," disables critical infrastructure, including traffic lights and communication networks, leading to dangerous situations across the city. Natsumi and Miyuki, along with their colleagues, are tasked with restoring order while uncovering the identity and motives of the hacker. The investigation reveals that Phantom is targeting the police as part of a personal vendetta, with ties to a past incident involving corruption within the force.

As the story progresses, the officers must navigate a series of high-stakes challenges, including car chases, bomb threats, and cyberattacks. Natsumi's impulsive and headstrong nature contrasts with Miyuki's calm and methodical approach, creating a dynamic partnership that drives the narrative forward. Supporting characters, such as the stoic Sergeant Ken Nakajima and the tech-savvy Yoriko Nikaido, play crucial roles in the investigation, adding depth to the ensemble cast.

The OVA explores themes of justice, loyalty, and the ethical dilemmas faced by law enforcement. The resolution of the story hinges on the officers' ability to work together, leveraging their individual strengths to outwit Phantom and restore public safety. The narrative maintains a balance between action-packed sequences and character-driven moments, ensuring continuity with the tone and style of the broader "You're Under Arrest" series.