In the fourth installment of the fighting franchise, Boyka is shooting for the big leagues when an accidental death in the ring makes him question everything he stands for. When he finds out the wife of the man he accidentally killed is in trouble, Boyka offers to fight in a series of impossible battles to free her from a life of servitude

Russian inmate Boyka, now severely hobbled by the knee injury suffered at the end of Undisputed 2. No longer the feared prison fighter he was, he has declined so far that he is now good only for cleaning toilets. But when a new prison fight tournament begins - an international affair, matching the best fighters from prisons around the globe, enticing them with the promise of freedom for the winner - Boyka must reclaim his dignity and fight for his position in the tournament.

A westerner named Casey, studying Ninjutsu in Japan, is asked by the Sensei to return to New York to protect the legendary Yoroi Bitsu, an armored chest that contains the weapons of the last Koga Ninja.