The story centers on middle school student Tsunayoshi "Tsuna" Sawada, who finds out that he is the heir to the most powerful Mafia family, the Vongola. The current Vongola boss thus sends his most trusted member, the titular character and infant hitman, Reborn, to train the future boss. Tsuna gradually becomes a more confident person and leader, while Reborn slowly recruits members into Tsuna's own Mafia family, which mostly includes Tsuna's schoolmates. Though they encounter new allies in the form of other hitmen and mafiosi, they also attract attention from those who want to annihilate the future boss of the Vongola Family. Enemies such as the Kokuyo Gang, a group of Mafia criminals, and The Varia, an elite team of assassins, prove to be formidable challenges for Tsuna's new "family". When the story later shifts into an alternate future, nine years and ten months later, in which some of the characters are transported into, they have to adapt to a world where every Vongola member, as well as their allies, is being hunted down by a powerful rival family of that era, the Millefiore. The characters transported from the past into this future are joined by adult versions of the ones who have yet to be replaced with their younger.