Loose Senki: Old Boy

Drugged and confused, Shinichi Gotou wakes up in a small room inside a private prison. Despite his pleadings, none of the guards will tell him who kidnapped him or why he is being held captive. As the days go by, his forced isolation slowly takes a toll on his sanity. He finds an outlet through training his mind and body for the day he will be able to wreak vengeance.

After 10 years, Shinichi is finally free. Once outside, he encounters a much changed world. His long imprisonment ripped him from society and kept him from having the normal life he desired. With nothing to lose, he begins his hunt for the one responsible. However, it seems that the person behind Shinichi's captivity is not done with him just yet, and thus begins a twisted game where only the winner survives.