Tsubaki is an undercover investigator who infiltrated the Soga gang to destroy the headquarters, the Shintou Group, as a whole. His plans came to a halt when the successor of the Soga gang, Kajiro, saw right through Tsubaki. Rather than throw him out to the dogs, Kajiro let him determine his outcome. Meanwhile, Tsubaki finds out that the Senior Superintendent of the police department and the head of the Soga gang were in collusion, driving Tsubaki to kill them both in a spontaneous frenzy. This awoke a hidden lust for blood in him that Kajiro accepted with open arms. Tsubaki chose to stay by Kajiro's side, but his role as an undercover cop is still ongoing... (Source: Futekiya)

Note: The extras called 'Charmed Straight to Hell (とろけておとして地獄まで)' are on DLsite. There are 4 extras and currently 3 are translated into English.