"He is only a DEKA but..." centers on the life of Shunpei Tachibana, a young police officer who works as a DEKA, a term referring to a member of the Tokyo Metropolitan Police Department's riot squad. Despite his position, Shunpei is often underestimated due to his small stature and unassuming appearance. However, he possesses exceptional physical strength and combat skills, which he frequently employs to protect civilians and uphold justice. The series explores his daily challenges as he navigates the complexities of law enforcement, balancing his duty with his personal struggles and relationships.

The narrative delves into the dynamics of the riot squad, highlighting the camaraderie and conflicts among its members. Shunpei's colleagues, including his superior officers and fellow DEKA members, play significant roles in shaping his journey. The series also introduces various antagonists, ranging from petty criminals to organized crime syndicates, who test Shunpei's resolve and abilities. Each case or mission he undertakes reveals more about his character, his unwavering sense of justice, and his determination to prove his worth despite societal prejudices.

Themes of perseverance, integrity, and the importance of teamwork are woven throughout the story. Shunpei's growth as a police officer and as an individual is depicted through his interactions with both allies and adversaries. The series balances action-packed sequences with moments of introspection, offering a multifaceted portrayal of life within the riot squad. Through its episodic structure, "He is only a DEKA but..." provides a detailed exploration of the challenges faced by law enforcement officers, while maintaining a focus on Shunpei's personal and professional development.