Gungnir, the deadliest assassin known in the underworld, decides to retire for the woman he loves. But after completing his final mission, betrayal strikes, leaving him and his lover caught in a deadly ambush. Miraculously surviving, Gungnir awakens in the body of an unfamiliar high school student. Determined to uncover the truth and claim happiness in this second chance, the former assassin begins a new battle for justice and redemption.