The Battling Angel (2003)

Police struggle to uncover Klein, a powerful criminal organization. Asuka, a young detective, is assigned to contact and escort Hatsumi, a mistress of the drug lord Kimikuni, but they are ambushed, leaving Hatsumi brain-dead and Asuka near to death. However, the police concoct a top-secret plan to infiltrate Klein by transplanting Asuka's brain into Hatsumi's body.