The Bogus Policeman (1967)

Toramatsu is a very enthusiastic policeman. Believing that the police should help others he is dismissed the force when his pistol is stolen while doing one of his good deeds. He saves a child from drowning, restores it to his mother, then finds that someone has run off with his gun and later uses it to kill. Then he breaks up a group of hoodlums attacking a young girl named Mieko, and the police, his former friends, deciding he has gone too far, decide to sue him. On top of this the girl gets hurt and, consequently, a child from the kindergarten where Mieko teaches is kidnapped on his way home. Impersonating a policeman he tracks down the gangsters and rescues the boy. Due to this he also uncovers a scandal which allows the police to make a long due clean up in the political world. As a reward for all of this he wins the regard of the girl but now faces a stiff prison sentence for impersonating the policeman he had been and which, at heart, he truly remains.

Unfortunately the movie The Bogus Policeman is not yet available on HBO Max.