A baby hitman named Reborn, who is the Vongola family's #1 hitman of the mafia ventures to Japan where he meets a boy named Tsuna who has no determination in life. Reborn becomes Tsuna's "tutor", while he attempts to train him to be the next boss of the Vongola family. Reborn also possesses a special bullet call the 'Dying Will' bullet, that is a specialty of the Vongola family. This bullet will revive you from death if you have any regrets when dying. When the bullet revives you, it gives you the determination to do what needs to be done. Source