Setsuna is a man of action, so the very next day, he sought out the Third Hokage to request permission to learn the Flying Thunder God Technique (Hiraishin no Jutsu).
Even though the Hiraishin is an S-rank jutsu, it's not actually a Forbidden Jutsu (Kinjutsu). Forbidden jutsu either drain the user's life force, cause severe, potentially fatal harm, or are simply too dangerous. The Hiraishin doesn't fall into that category.
Therefore, Setsuna simply needed to contribute enough to the village. After reviewing his request, the Third Hokage determined that Setsuna's recent heroic effort in repelling the Cloud Village ambush was more than enough to cover the cost.
With the Hokage's approval, Setsuna quickly approached Minato (the Fourth Hokage). Minato was, of course, happy to teach him.
They met at Training Ground Three, where Setsuna usually practiced, and Minato confidently began to give pointers.
"The Flying Thunder God Technique is actually quite simple," Minato said, counting on his fingers. "First, you inscribe the Hiraishin Formula (Jutsu-shiki). Second, you sense the formula. And third, you instantly teleport to it."
Setsuna waited a moment, but when Minato didn't continue, he couldn't help but ask, "That's it?"
That question seemed to remind Minato. He clapped his hands and added, "Oh! Before you teleport, you must always prepare a plan in your mind. Otherwise, if you arrive and freeze up, you'll just be handing the enemy an opportunity."
Setsuna fell silent, briefly wondering if Minato was messing with him.
But looking at Minato's clear, sincere eyes, and recalling the sometimes ridiculous names he gave his own jutsu, Setsuna sighed. Okay, he thought. It looks like Minato doesn't really have a natural talent for teaching.
Setsuna asked humbly, "I believe I can manage inscribing the formula and sensing it. But how exactly do I go about the instantaneous teleportation part?"
Minato blinked, seemingly confused by the question. "If you can sense it, can't you just teleport to it? It's like your brain sending a command to your body—you just follow the instructions!"
Setsuna sighed again. Is this the way a genius practices? he wondered. My apologies for interrupting.
If all else fails, he thought, when I'm strong enough to fight aliens, I'll just go steal the God Tree and plant it. The God Tree fruit grants spatial abilities, eternal life, and an immortal body. Even Frieza would be drooling.
Setsuna nodded and said, "I think I get the general idea. I can handle the rest of the training on my own."
Minato saw nothing wrong with this and earnestly replied, "The Hiraishin truly isn't hard. You just need to practice a lot."
After Minato left, Setsuna put the Hiraishin out of his mind and focused entirely on his current training.
Not long after, he heard Guy's voice calling from outside the training ground.
"Setsuna-nii-san, I knew I'd find you here!"
"Hey, Guy, what's up?"
"Setsuna-nii-san, let me introduce you. This is my father, Might Duy…"
Before Guy could finish, Duy seemingly Flashed in front of Setsuna, speaking with great sincerity. "Little Setsuna, thank you so much for looking after Guy. To show my gratitude, I want to pass on the only technique I know to you."
Duy was worried Setsuna might refuse. After all, he was just an unknown Genin (Lower Ninja). A normal person might not think he had anything powerful to offer, and a narrow-minded person might even find the offer insulting.
"Oh? Thank you so much, Uncle Duy!" Setsuna naturally wouldn't refuse.
He had been wondering how to politely ask to be taught, and here was Might Duy offering to teach him the Eight Gates Formation (Hachimon Tonkō) of his own accord. What a pure soul!
Of course, Setsuna didn't feel guilty. He was genuinely teaching Guy how to train his "Chi".
Might Duy was even more touched than Setsuna, seeing pure respect in the young man's eyes.
"Watch closely! This is the Eight Gates Formation!" Duy clenched his fists, and a solid, visible aura of chakra burst from his body. It was hard to believe this was the amount of chakra a Genin could possess.
"Dad actually learned such an amazing move!" Guy was shocked, and also felt a little ashamed.
Though he believed in his father, he sometimes felt embarrassed by his low-profile existence, occasionally wondering if people like them could truly keep their flames of youth burning. Compared to his own doubts, his father had taken his belief and pushed it this far. His dad was the real strong one.
Duy only demonstrated it briefly before suppressing his power and seriously explaining the key points for training the Eight Gates.
Sure enough, there's something wrong with his body, Setsuna thought, instantly knowing. His body is full of old injuries and blockages. Even if he only opens the first three Gates, he can't sustain it for very long. This was likely why, in the original timeline, three of the Seven Ninja Swordsmen of the Mist were able to survive their encounter with Eight Gates Duy.
While Duy's physical base certainly couldn't compare to Guy's during the Fourth Great Ninja War, there would naturally be a difference in power when opening the Eighth Gate. However, even if the power was scaled down by a factor of ten or twenty, the full release of the Eighth Gate should still be an instant kill against the Swordsmen. The damage output should have been too great to overcome.
The fact that Duy opened the Eighth Gate and still let three of the Swordsmen escape was enough to prove that his body had already broken down.
It wasn't surprising. Guy had greater natural talent and was standing on the shoulders of his "giant" father, allowing him to avoid many mistakes. Duy, however, had no resources or guidance. He had to plug away at his training alone, and he had clearly injured himself in the process.
"So, have you both memorized the key points for opening the Gate of Life (Seimon) that I explained?" Duy asked.
"I have!" Guy nodded heavily, and then a chakra aura erupted around him.
"He learned how to open the Gate of Life after hearing it just once?!" Duy was stunned.
A moment later, a similar chakra aura erupted from Setsuna's body.
Duy's jaw dropped. Is this what genius looks like? Is my years of hard training nothing more than mere experience and practice for a talent like this?
Setsuna was quite emotional. It felt good to be back in an area he was familiar with to regain his confidence.
As for Ninjutsu training? Sorry, I'm not really good at it.
Duy was excited after his shock wore off. This was the meaning of legacy! He saw the future in these two boys.
"Alright, then let's move on…" Just as Duy was about to start explaining the Gate of Healing (Kyūmon), Setsuna interrupted. "Uncle Duy, would you mind letting me try to adjust your body?"
"Huh? Okay," Duy replied. Although he didn't understand Setsuna's intention, he didn't refuse the kind offer.
Setsuna quickly pressed and poked about a dozen points on Duy's body. Suddenly, Duy coughed up a large amount of dark blood.
Guy watched, alarmed, and rushed to ask, "Setsuna-nii-san, what's happening to my dad?"
Setsuna explained, "Those are the internal injuries your father accumulated during his Eight Gates training."
Duy straightened up and swung his arms, a look of shock on his face. "My body feels so much lighter! I thought this stiffness was just me getting older!"
"That's great! Thank you, Setsuna-nii-san!" Guy sighed in relief, then asked, "Is Dad completely healed now?"
Setsuna shook his head. "Not yet. I only used Point-Pressing (Tenketsu) to force the old blockages out of his system. But to truly fix the depletion in his body, he needs a medicinal meal supplement."
"A medicinal meal? How do we make that?" Guy asked, confused.
"Don't worry. Just leave it to me," Setsuna said confidently. He was, after all, a Gourmet Hunter certified in the Gourmet Universe, and he had a decent understanding of medicinal cuisine.
In the Gourmet Universe, he might not even qualify as a kitchen assistant, but in the Naruto Universe, they could call him Chef Setsuna.
"Oh, you don't have to do that! That's too much trouble," Duy said, feeling embarrassed. He had come to repay a kindness, but now he was receiving even more.
Setsuna waved his hand. "Not at all! Guy and I need to prepare medicinal meals for our training anyway. It's no trouble at all to add one more."