Joji "Jo" Mizuno (Jo Shishido) comes into a sleazy nightclub run by the Nomoto gang. He runs up a huge bill, and when he refuses to pay, is hauled into the Nomoto gang's office. After he proceeds to beat up one of the mooks who dragged him in, gang boss Tetsuo Nomoto hires him on the spot. He goes to work as an effective enforcer for the Nomoto gang, running interference against the Sanko gang. However, once Jo learns that the Nomoto gang has a huge drug buy in the works, he goes to the Sanko gang and offers to betray his own gang.
Why? Because Jo has hidden motives. It turns out that he's not really a gangster after all. He is in fact an ex-cop who went to jail when a Cowboy Cop operation of his went awry. While he was away his old partner Takeshita killed himself...or so the police think. Jo is sure that Takeshita was actually murdered and that someone in the Nomoto gang was responsible. He has vowed to find out who did and to "kill the killers".
This film contains examples of:
- Arch-Enemy: Jo has Nomoto Tetsuo, who killed his partner.
- Bad Guys Do the Dirty Work: Jo swore to "kill the killers", Jo flinches at the prospect when he confronts Kumiko, the real killer. So instead, he tricks her into pressing Hideo's Berserk Button by getting her to mention, in Hideo's earshot, that Hideo is the Son of a Whore. Jo then leaves just as Hideo is busting out his razor to use on Kumiko.
- Berserk Button: Don't call Hideo's mom a whore or he'll mutilate your face.
- Big Bad: Nomoto Tetsuo, the Yakuza boss who killed Jo's partner.
- Brandishment Bluff: Jo gets the drop on the Sanko gang when they came to kill him by pointing a shotgun at them. The gang didn't know that the shotgun's action was broken.
- Briefcase Full of Money: The Nomoto gang brings one to a drug deal. Joe gets the Sanko gang to steal the money after it's handed over, in an effort to start the two gangs fighting each other.
- Clothing Damage: An enraged Nomoto whips Keiko until she's half-naked. Then, because he's a sadist and it turns him on, they have sex.
- Cowboy Cop: In the backstory. Jo embezzled money from the police force to investigate a hunch of his. Then he tortured a suspect. In a Surprisingly Realistic Outcome, this landed him in prison for embezzlement and assault.
- The Dog Was the Mastermind: Who is Nomoto's "sixth mistress"? Who really killed Takeshita? Why, it was his sweet, kindly wife Keiko! It turns out that Keiko was Nomoto's minion all along, and head of his prostitution ring. She married Takeshita to keep tabs on him, and killed him when he found out about her.
- Fingore: Nomoto tortures Jo by sticking a knife under his fingernails, after finding out that he used to be a cop.
- Flashback: A flashback shows Jo explaining to Takeshita that he wasn't really embezzling, but instead taking money to use in an off-the-books Cowboy Cop investigation.
- The Infiltration: Jo worms his way into the Nomoto gang so he can destroy it, and specifically so he can find out who killed his partner.
- Monochrome to Color: Given that it was 1963 it isn't that surprising when the opening scene, with the cops finding the bodies of Takeshita and his girlfriend, is in black and white. That sets up a big punch when the movie cuts away from that scene to a shot of a noisy, chaotic Red Light District in vivid color.
- Never Suicide: The film opens with a scene of the cops discovering the body of Detective Koichi Takeshita, who has killed himself along with his lover. In fact they were both murdered.
- Parking Garage: The Sanko gang ambushes Jo in the parking garage of his apartment building, but he turns the tables and gets the drop on them.
- Pay Evil unto Evil: Jo's avowed purpose. Not being a cop anymore, he seeks to "kill the killers" responsible for his partner's death.
- The Reveal: Takeshita's real killer wasn't any of the Yakuza goons, it was his wife Kumiko, who was working for Nomoto all along.
- Right-Hand Cat: Nomoto, the boss of the gang, has a cat he's always carrying around and snuggling with. In his first scene he throws a knife at Jo with one arm while holding the cat with another.
- Shoot the Rope: An unusual example. Jo has been strung up by his feet from a chandelier. Hanging upside down, he eventually frees himself by swinging until he can reach a gun on a table, then shooting the rope.
- Son of a Whore: Younger brother Hideo Nomoto is the son of a streetwalker. He gets homicidally angry at anyone who reminds him of this.
- Victoria's Secret Compartment: Jo stuffs some money down the top of one of the hookers he's cavorting with at the nightclub. It's all part of a plan to make a big show and get the Nomoto gang to notice him.
- The X of Y: Youth of the Beast. It's a completely random title that has nothing to do with the story.
- Yakuza: A tale of two rival Japanese crime gangs and the renegade ex-cop who sets them warring against each other, in an effort to destroy them and avenge his dead partner.
- You Can Leave Your Hat On: A dancer wearing nothing but bikini bottoms and pasties is the entertainment at the nightclub that serves as the Nomoto gang's base.