Ichiro Inuyashiki is down on his luck. While only 58 years old, his geriatric looks often have him written off as a pathetic old man by the world around him and he’s constantly ignored and disrespected by his family despite all that he’s done to support them. On top of everything else, his doctor has revealed that he has cancer and it appears that he has little time left in this world. But just when it seems things couldn’t get any worse, a blinding light in the night sky strikes the earth where Ichiro stands. He later wakes up to find himself unscathed, but he soon starts to notice that there’s something…different about himself.
Like his brother Baki , he has since childhood wished to surpass his father Yujiro Hanma in strength and martial ability. His obsession to defeat his father led to extreme overtraining at various fighting gyms which led to the deterioration of his body. He wasn't much more than skin and bones because his body had been damaged by overwork. For example, Jack would punch sandbags for over 12 hours without eating, resting, or even using the restroom. He claimed he will never be able to defeat his father with normal training. After his encounter with the Yujiro-inspired scientist named Dr. John who witnessed Yujiro killing a polar bear. The scientist promises him great results if he tries using a special drug. He received from him a concentrated formula of steroids to match his overtraining and gained immense power in the process and Jack had become even more mentally unstable.