"Cells at Work! Code Black" is a spin-off of the original "Cells at Work!" series, presenting a darker and more intense portrayal of the human body's cellular functions. The narrative unfolds within the body of an unhealthy individual, whose lifestyle choices—such as smoking, excessive drinking, poor diet, and lack of exercise—create a hostile and deteriorating environment for the cells. This setting contrasts sharply with the healthier body depicted in the original series, emphasizing the consequences of neglect and abuse on biological systems.

The protagonist is a red blood cell, designated AA2153, who navigates the chaotic and overworked circulatory system to deliver oxygen and nutrients. The body's declining health results in constant emergencies, such as blocked arteries, infections, and organ failure, forcing the cells to operate under extreme stress. AA2153 encounters various challenges, including navigating through hardened blood vessels, dealing with oxygen shortages, and witnessing the deaths of fellow cells due to the body's poor condition. The red blood cell's journey highlights the interconnectedness of bodily systems and the fragility of life when subjected to harmful habits.

Other key characters include a white blood cell, designated U-1196, who fights off pathogens and infections while grappling with the overwhelming workload caused by the body's weakened immune system. Platelets, macrophages, and other cell types also play crucial roles, each struggling to maintain functionality in the face of mounting crises. The narrative frequently explores themes of perseverance, teamwork, and the harsh realities of bodily decline, as the cells strive to keep the body alive despite increasingly insurmountable odds.

The story delves into specific medical conditions and their impact on cellular life, such as erectile dysfunction, liver cirrhosis, and heart disease, often using these scenarios to illustrate the broader consequences of unhealthy living. The tone remains grim and urgent, with moments of hope and camaraderie among the cells serving as a counterbalance to the pervasive sense of despair. Through its depiction of a body in crisis, "Cells at Work! Code Black" offers a stark and educational perspective on the importance of maintaining health and the intricate workings of human biology.