Hidden Bullet: Meeting a Dangerous Boy is a bonus short novel in the Reborn! fanbook, Vongola 77.
After hearing news that the Vongola's 9th Boss collapsed from a sudden illness, Dino visits the Vongola headquarters to see him, but was surprised when told that the ninth Boss had recovered and is fine now, but even more surprised when the old man refused to meet him. He had no choice but to leave, despite his misgivings. In the garden outside the headquarters, Dino meets a member of the CEDEF, Oregano, in secret and is informed that Iemitsu wants to talk to him about something.
Back in the headquarters, Leviathan reports to Xanxus that Dino had left without suspicion. Then, Levi gets into a squabble with Belphegor, as usual, until they're stopped by Lussuria. Mammon reports that the "other thing" is ready, and Squalo brings in Gola Mosca. Xanxus introduces his squad to their newest member.
Elsewhere around midnight, Basil and Lal Mirch meet Turmeric and Iemitsu in a warehouse at the port. Basil reports that he has completed the reconnaissance. As they suspected, the Varia is searching for Iemitsu, who surmises that they're after the Vongola Rings; specifically, the Half Vongola Rings kept by Iemitsu. They know Xanxus is after the Vongola Boss poistion, and the Varia have already begun to look for the other boss candidate in Japan. But, Iemitsu has yet to give up on the other young candidate, and he spares a thought for Nana and Tsuna.
Meanwhile, in the shopping street in Namimori, Tsuna runs into Haru, Kyoko and Fuuta in the coffee shop. Tsuna is worried about Fuuta, whose ranking abilities have been crippled since the incident with Mukuro, but Fuuta assures Tsuna that he is all right but Tsuna realizes that Fuuta was still traumatized by the incident. Suddenly, Tsuna feels a sharp sense of the chills but it's gone in a second, leaving him to wonder what it could have been. It seems that Fuuta felt it too, but before they could think further about it, the girls take Fuuta out of the shop to make him feel better. Tsuna is about to follow them but outside, he catches sight of a little boy crying alone in a corner and Tsuna can't help but feel sorry for him. So, Tsuna decides to help him look for his mother. On their search, he runs into Longchamp and his new girlfriend. However, when Tsuna is talking to Longchamp, he suddenly shivers with the chills again, and hears a voice. Tsuna screams in terror, surprising Longchamp. Tsuna asks him if he heard any voices and Longchamp replies that he didn't hear any voice, so Tsuna dismisses it as his imagination, and continues to help the little boy find his mother.
Tsuna and the boy leaves the shopping street and enters the residential district. Although the boy seems certain that he'd be able to find his mother in this direction, Tsuna is not so sure. Still, he follows along. Suddenly, he hears someone yelling. It's Gokudera being chased by Bianchi who has been delivering her Poison Cooking to Gokudera every day since the Kokuyo incident to help him nurse his injuries, apparently. Gokudera begs Tsuna to help him, but Tsuna is afraid and reluctantly lets Bianchi chases Gokudera.
As Tsuna sympathetically watches Gokudera run away with Bianchi chasing after him, he (Tsuna) suddenly feels the little boy's hand on his neck, and relaxes a little. Tsuna asks if the little boy is tired, and offers to carry him on his back, but the boy's fingers remain on Tsuna's neck like points of ice, and for a moment, Tsuna feels as if something icy cold had been poured into him.
Once again, someone yelling from far away, distracting Tsuna. The boy also releases his fingers from Tsuna's neck. This time, it's Doctor Shamal being chased by Ryohei. Ryohei was badly injured by Ken a month ago. He'd heard that Shamal was a very good doctor and wanted Shamal to examine him so that he could recover faster; however, Shamal doesn't see male patients, and Ryohei wouldn't take no for an answer. So, Shamal tells Ryohei that Tsuna is actually an unlicensed doctor because he has this secret technique so Tsuna should take a look at Ryohei. Stupidly, Ryohei believes Shamal, who makes a good escape, and Tsuna spends over an hour trying to clear the misunderstanding.
Afterwards, Tsuna and the boy end up in riverbank, feeling exhausted. Tsuna tries to cheer up the boy by assuring him that they would find his mother. However, the boy only laughs and says don't bother with it anymore. Tsuna feels that something is wrong with the boy but is interrupted when a ball hits his head, which turns out to be Yamamoto's ball. Yamamoto apologizes and recognizes the boy as the little brother of one of his team mates. When they were about to take him, Hibari interrupts and asks Tsuna if he wants to practice with them. Yamamoto explains that he asks Hibari for help with their practice by saying that if they won the upcoming tournament, the name of Namimori will be known throughout the country, much to the surprise of Tsuna, who was then chased by Hibari.The boy then murmurs that it was "another failure" as he watches Tsuna being chased by Hibari with the word "Six" shown clearly in his right eye. Shortly after, he is picked up by his brother and they go home. It is revealed that Mukuro just barely managed to match the boy's spiritual wave, so he couldn't incarnate his existence into reality and it felt like his power was continuously fluctuating; however, there was one advantage. Because he couldn't achieve a complete assimilation, he could escape detection by Tsuna, who possessed a "Hyper Intuition" ability. However, by possessing an innocent child, he could get much closer to his target.
The next day, Tsuna and the others watch Yamamoto's baseball tournament. Mukuro, who's also watching by possessing the same boy, thinks about his plan yesterday: He had intended to lead his target to a deserted location, but in the end, he had miscalculated his plans due to Tsuna's friends' interference.
As he thinks this, he looks at Tsuna and the others who are cheering for Yamamoto. He then thinks of how Tsuna is surrounded by many of his friends, looking very happy and laughing along with them. Not just for this. Wherever Tsuna went, his friends are sure to be around him. If not for them, he would definitely have succeeded in killing him. At this moment, he suddenly remembers Ken and Chikusa, who are still on the run from the Vindice. Suddenly, however, he feels a wave of dizziness, and presses his hand to his face. He realizes that he is near his limit in using the boy's body and decides that while he may not need friends, but "she" probably will.
At the same time, in a place faraway from Japan, Ken, who has been lying down, jumps up with a gasp as he wakes up and immediately wakes up Chikusa, asking him if he saw 'it' too. Chikusa awakes and says that he also saw Mukuro in their dream, saying that they need to find the other him. Even though confused at what it means, they decide to go to Japan. Back in Japan in a hospital room, Nagi awakens and touches her right eye, quite surprised that she's alive. She realizes that Mukuro is real and not a dream. The story ends with Nagi uttering Mukuro's name as she cries in happiness.