Gungnir is the strongest assassin, known throughout the underworld. He decides to retire for the sake of his girlfriend, to whom he has pledged his future, but on the night he completes his final job, his partner betrays him and he and his girlfriend are shot to death. He somehow manages to survive, but when he wakes up, he finds himself as a strange high school student! He must uncover the truth and find happiness in his second life. This is the start of a battle action drama starring the former strongest assassin.