Once the city's strongest assassin, Nagi Taira is left unresponsive after protecting his younger sister, Shizuka, and now lives in a wheelchair. Working as an assassin broker, Shizuka takes on assassination jobs and brings her brother to the battlefield for his "rehab." Hoping to fully awaken Nagi, Shizuka accepts a contract to take out Goro Arashiyama, the boss of a notorious robbery gang.

For Nagi's rehabilitation, Shizuka searches for someone with an even stronger killing intent and sets her sights on Ryo Takanashi, a highly skilled sniper. Takanashi is a deranged man interested only in "good targets." Shizuka asks him to shoot Nagi, hoping his intense killing intent will fully awaken her brother.

Naozumi Kurosaka and his younger sister Touko, students at the assassin-training institution "the Academy" Nagi and Shizuka once attended, arrive on Guicheng Island in pursuit of them. Members of Class A, they suspect Nagi is behind the spate of strange killings on the island. When Shizuka runs into them, they grill her about Nagi's whereabouts, and she finds herself in grave danger.

The Kurosaka siblings track down Nagi and Shizuka, leaving them cornered. Determined to use even this situation as part of Nagi's rehabilitation, Shizuka provokes Touko, triggering a fierce battle between Nagi and her. Nagi fights in his Tank Chair Mark III: "Iron Fists" but is soon forced into a tough battle when Naozumi joins the fray.