"Double Decker! Doug & Kirill" is a sci-fi action-comedy anime set in the fictional city-state of Lisvaletta, where crime and illegal drugs, particularly a dangerous substance called "Anthem," plague society. The story revolves around the Special Crime Investigation Unit "SEVEN-O," a unique police division tasked with tackling high-profile criminal cases. The unit operates with a two-tiered system: the "Double Decker" approach, where seasoned detectives (the "Deckers") are paired with rookie officers (the "Bait") to solve cases.

The narrative primarily focuses on the partnership between Doug Billingham, a laid-back yet highly skilled veteran detective, and Kirill Vrubel, a naive but enthusiastic rookie with a mysterious past. Kirill joins SEVEN-O under unusual circumstances, and his arrival sparks a series of events that uncover deeper conspiracies within the city. As the duo investigates Anthem-related crimes, they encounter a colorful cast of characters, including their eccentric colleagues in SEVEN-O, such as the sharp-witted leader Travis Murphy, the tech-savvy Max, the stoic Yuri Fujishiro, and the energetic rookie Kay.

The plot intertwines episodic crime-solving with an overarching mystery surrounding Kirill's origins and the true nature of Anthem. Each case reveals more about the city's underworld and the shadowy figures manipulating events from behind the scenes. The series balances humor, action, and intrigue, with the dynamic between Doug and Kirill serving as the emotional core. Their contrasting personalities—Doug's pragmatic cynicism and Kirill's idealistic optimism—create a compelling partnership that evolves as they face increasingly dangerous threats.

As the story progresses, the stakes escalate, culminating in a confrontation with the enigmatic organization responsible for Anthem's distribution. The series explores themes of trust, justice, and the blurred lines between good and evil, all while maintaining a lighthearted tone through its quirky characters and comedic moments. "Double Decker! Doug & Kirill" delivers a mix of episodic adventures and a serialized narrative, offering a blend of mystery, action, and camaraderie within its unique sci-fi setting.