"Kateikyoushi Hitman Reborn!" centers on Tsunayoshi "Tsuna" Sawada, a clumsy and academically struggling middle school student who is deemed unfit to inherit his family's legacy. His life takes a dramatic turn when Reborn, a hitman from the powerful Vongola Mafia family, arrives as his home tutor. Reborn, a baby with a fedora and a talking chameleon named Leon, is tasked with training Tsuna to become the next Vongola boss. Using a special "Dying Will Bullet," Reborn unlocks Tsuna's latent potential by triggering a state of heightened determination and strength, allowing him to face increasingly dangerous challenges.

The narrative unfolds in two distinct arcs. The first half of the series is episodic and comedic, focusing on Tsuna's daily life as he reluctantly adapts to Reborn's unconventional training methods. During this time, Tsuna forms bonds with classmates who later become his loyal guardians, each representing a specific element of the Vongola family: Hayato Gokudera (Storm), Takeshi Yamamoto (Rain), Ryohei Sasagawa (Sun), and Kyoya Hibari (Cloud). These relationships are tested as Tsuna confronts various adversaries, including rival mafia factions and internal threats to the Vongola family.

The second half shifts to a more serious and action-driven tone, delving into the mafia underworld. Tsuna and his guardians face off against the Varia, a group of elite Vongola assassins, in a battle to determine the rightful heir to the family. This arc introduces deeper lore about the Vongola's history and the significance of their rings, which symbolize their authority. Later, the story explores the conflict with the Millefiore family, led by Byakuran, who seeks to dominate the mafia world using powerful artifacts known as the Tri-ni-set. Tsuna and his allies must travel through parallel timelines and harness advanced techniques like the X-Burner to protect their future.

Throughout the series, Tsuna evolves from a reluctant and insecure teenager into a capable leader, driven by his desire to protect his friends and uphold the Vongola legacy. Reborn's mentorship, combined with the unwavering support of his guardians, shapes Tsuna's growth as he navigates the complexities of the mafia world. The series blends humor, action, and character development, creating a narrative that balances lighthearted moments with high-stakes battles and emotional depth.