The series starts in the middle of Italy as a small child is seen walking the streets of the city, wearing a suit and with a chameleon on hit hat. He arrives at a bar to announce to a group of men that the leader of the Vongola Mafia has decided on the next heir and that he is to travel to Japan to find the new candidate. The men at the bar are shocked as a man from the streets attempts to stab the small child. The chameleon shifts into a gun as the baby announces that he is the hitman Reborn.
In Japan, Reborn walks through the early morning streets as he drops something off in a mailbox before walking off. Nana Sawada checks the mail as she finds a letter for her son from Reborn, saying that he will be tutored to be the leader of the next generation. As she goes to wake her son, she finds Tsunayoshi (Tsuna) still asleep as she goads him to wake up or else he'll be late. Upon further examination, she finds that he got a 15 on his last math exam, scolding him for it as he promptly wakes up. He tries to push it off as being a failure at math as Nana announces that someone will be tutoring him. Saying he doesn't need a tutor, he realizes he is late and falls down the stairs, only to meet Reborn. The young child tells Tsuna that he is there to tutor him as Tsuna laughs at how Reborn is nothing but a child, causing him to get beat up by the kid. Nana comes downstairs to find Reborn as well, laughing it off that he is Tsuna's tutor before realizing he is serious. Tsuna remembers he is late as he runs off.
On his way to school. Reborn rides on his head as he reveals that he knows a lot about Tsuna, from his poor performance in school to his nickname of "No Good Tsuna" as he is scared of even small dogs. As Reborn tries to tell him he is a Hitman from the Mafia, Kyoko Sasagawa approaches the two, commenting on the cuteness of Reborn who thinks his story about being in the Mafia is funny. As she walks off, Tsuna sulks about who he can't confess to her his true feelings for her. Reborn has a solution as he shoots Tsuna in the head. As he dies, he voices his regrets for not telling Kyoko how he felt. After collapsing, he is somehow revived, losing his clothes and goes to run off to profess his feelings for Kyoko. Things don't go so well as he botches the love confession, not to mention he is nearly nude in public. Kyoko runs off as she is speechless on what happened. Tsuna tries to put together what happened as Reborn pops out with his mended clothes and explains the Dying Shot.
The Dying Shot is a bullet that kills a person but will revive them if they have any regrets, becoming powerful and determined to fulfill said regret. Tsuna points out that its no good if you don't have any regrets when dying, which Reborn asserts: If you are shot with the Dying Bullet and have no regrets, you will not be reborn. As he scrambles to get his stuff together as the late bell rung, the head of the disciplinary board chastises him for his behavior as he runs off to class. When he arrives, everyone mocks Tsuna for confessing his love for Kyoko in the nude, Kyoko silent as someone tells Tsuna that the captain of the Kendo Team has challenged him to a fight after school. Tsuna tries to flee, but Reborn tells him that standing up and losing will make him a much better man than simply running away.
After school, the captain sits around as he waits for Tsuna. He knows he has it in the bad as the referee is rigged to not award Tsuna any points whatsoever. Everyone thinks Tsuna chickened out but he does arrive, which makes people think slightly differently. The captain announces that he will avenge Kyoko and whoever wins will get her, which makes her flustered. The two fight as Tsuna is disarmed as has no means of fighting back. However, Reborn is able to get a shot in with the Dying Shot, causing Tsuna to regret not being able to win the fight. He is revived, albeit it in his underwear again, as he proceeds to beat the kendo captain, tearing out all his hair in the process. The Shot wears off as everyone is stunned that he won. Kyoko approaches him and instead of making fun of him or rejecting him, she reveals she thinks he is cool and claims to call him Tsuna-kun from now on.
Back at home, Reborn reveals that his real reason for being in Japan is that Tsuna is the great-great grandson of the Volga crime family and that the young child is there to train him to take the head of the Mafia. Tsuna is reluctant to agree as Reborn takes his bed and booby traps it, claiming that he does not like to be awakened at night. As Tsuna laments his situation, Nana bids Reborn good night, asking him to look after her son.