Mr. Brain follows the story of Tsukumo Ryusuke, a quirky and eccentric neuroscientist who works for the National Research Institute of Police Science. After a freak accident leaves him with a unique perspective on the human brain, he uses his profound knowledge of neuroscience to solve complex criminal cases that baffle the regular police force. His unconventional methods and obsessive curiosity often put him at odds with his colleagues, but his ability to decode the biological triggers behind human behavior proves indispensable. As Tsukumo delves into the minds of criminals, he is assisted by his dedicated assistant, Yuri Kazune, and works alongside detective Keichiro Hayashida. Together, they navigate the intricate web of criminal psychology, uncovering the hidden motives and neurological secrets that drive people to commit heinous acts. The series blends procedural mystery with scientific exploration, highlighting the fascinating and often dark complexities of the human mind.