Kakashi Gaiden

The originally appeared as a six-chapter spin-off of the Naruto manga. It takes place several years before the start of the series, but acted as an intermission between Part I and II of the series. It was later adapted into two Shippūden episodes: 119 and 120.

The story surrounds a 13-year-old Kakashi Hatake who embarked on a dangerous mission with his team, which included another boy named Obito Uchiha, from the same clan as Sasuke Uchiha, a medical-nin kunoichi named Rin, and their teacher, the future Fourth Hokage, Minato Namikaze.

This arc answered many questions about Kakashi's past. It was learned how Kakashi received his famous Sharingan in his left eye, how he got his equally famous scar over his left eye, the identity of his childhood teammates Obito and Rin, when Kakashi became a Jōnin, and the reasons behind his current personality. It also gave a glimpse of what Minato was like when he was still alive. Portions of it were re-used as flashbacks in Kakashi's reflections in the movie Naruto Shippūden 3: Inheritors of the Will of Fire.

Synopsis
In the midst of the Third Great Shinobi World War, at a team meeting to hear about their new mission, Kakashi received gifts of a special kunai from Minato and a medical pack from Rin for his advancement to Jōnin. His other teammate, Obito, in addition to being late (supposedly from helping an old woman), had forgotten to bring a gift, and became annoyed at Kakashi when he dismissed whatever he would've given him as useless. Obito boasted that, once he awakened his Sharingan, the kekkei genkai of the Uchiha clan, he would finally surpass Kakashi. Minato told his team their mission: they must travel through Kusagakure in the midst of a conflict between Iwagakure and Konohagakure to destroy a bridge so as to stop them from receiving their supplies. Along the way, the Konoha ninja noticed that they were being watched from the trees by Mahiru, an Iwagakure ninja. Kakashi saw this as a good time to try out his new jutsu, the Chidori, although Minato warned him about being hasty. Kakashi rushed off to attack their hidden foe. Mahiru, however, used the Shadow Clone Technique, creating multiple identical clones of himself that Kakashi uselessly attacked. Mahiru used the mayhem he had created to attack Obito, who was paralyzed by fright, but Minato rescued him from harm. Meanwhile, Kakashi was being attacked by the real Mahiru, who buried his katana deep into Kakashi's right shoulder. Kakashi narrowly escaped death with a well-timed diving tackle by Minato, who attached a special tag onto Mahiru's foot in the process. Assured of the team's well-being, and having transported them a safe distance away, Minato left Rin to care for Kakashi's wound, and instantly teleported behind Mahiru using his famed Flying Thunder God Technique. Mahiru was shocked to have faced, and, before he died, regretted that he hadn't obeyed the "flee on sight" order given by his village if anyone was unfortunate enough to encounter Minato.

Minato returned to find the team at each others' throats, with Kakashi calling Obito a coward for hesitating on the battlefield and the latter criticizing Kakashi for blindly rushing into battle. Minato told Kakashi to never again use the Chidori, because of its inherent flaw: Kakashi's eyes were not fast enough to see his opponent's counterattack, putting him in great danger. After criticizing Obito for speaking without thinking, Minato decided that rest was in order. Disturbed by the day's events and by Kakashi's abrasive behavior, Obito was unable to sleep, and sought out Minato to ask him why Kakashi acted the way he did. Minato told him of how Kakashi's legendary father, Sakumo Hatake, who had also been known as, had killed himself after the village had looked down on him for rescuing his teammates rather than completing his mission. From that point forward, Kakashi had adopted a mentality strictly adhering to the rules of the shinobi, which stressed the elimination of emotions and utter concentration on completing one's mission. Minato then urged Obito to try to understand Kakashi, and not to hate him.

After Rin used her medical abilities to heal Kakashi's wounds, Minato told the team that they had to split up. Minato headed off alone, leaving the group to be led by Kakashi. Obito, touched by Minato's words the night before, accepts Kakashi as the leader, surprising both Kakashi and Rin. Kakashi then led his team further into Rock territory in order to accomplish their objective. However two Iwagakure ninja, Kakkō and Taiseki appeared in search of Mahiru, and ambushed the trio. They managed to capture Rin before fleeing. Obito told Kakashi that he was going after Rin, but Kakashi, as the team leader, decided that they should complete their mission without detouring to protect a friend. Obito then punched Kakashi, telling him that his father had been a true hero when he had decided to protect his friends. Obito then went off on his own to save Rin while Kakashi walked the opposite way. Minato arrived to help the demoralized and heavily outnumbered Konoha ninja in battle by using his Flying Thunder God Technique to decimate the opposing army's numbers. Elsewhere in Kusagakure, Obito's words pressed heavily upon Kakashi's conscience, while Obito found out where the Iwagakure ninja were hiding Rin. But, as Obito plotted his plan of action, Taiseki, having observed Obito's appearance, sneaked behind Obito using an Hiding with Camouflage Technique. Before Taiseki was able to strike Obito however, Kakashi arrived, initially claiming that he had believed that Obito could not handle the situation on his own. He successfully deflected an attack that would have been otherwise fatal against Obito, but lost his left eye in the process. Obito was frightened and began to cry, but eventually regained his focus and activated his Sharingan for the first time. Using it, he was now able to see his invisible attacker and kill him.

Kakashi bandaged up his eye, and he and Obito approached the cave to save the weakening Rin from Kakkō, the remaining Iwagakure ninja. Using their skills and teamwork, and Obito's new Sharingan, they struck Kakkō in the shoulder and saved Rin. However, the battle had weakened the cave, and, in a last ditch effort to defeat the young shinobi, Kakko caused the cave to crumble in upon itself. With rocks falling all around the Konoha ninja, Obito threw Kakashi out of the way of a falling rock, which landed on him instead, and crushed the entire right side of his body. Mortally wounded, and knowing that his life was ending, Obito told Rin to remove his remaining left eye, and offered it - along with the Sharingan - to Kakashi, as a final gift and belated present to Kakashi for being promoted to Jōnin. Kakashi began to regret his previous attitude, believing himself unworthy to lead the group, but Obito told Kakashi that he was a great ninja, and that he could become Kakashi's eye and help them all live on. A short time passed, and Kakashi emerged from the rubble as a holder of the Sharingan, and a new scar.

When Kakashi emerged, he attacked Kakkō and managed to kill him using his newly perfected Chidori. More Iwagakure ninja arrived and caused a further cave-in. Rin and Kakashi were forced to leave Obito behind as the rubble enveloped him. Enraged, Kakashi went to fight his opponents. He activated his Chidori and prepared the special kunai given to him by Minato. Before the enemies attacked him, Kakashi threw the kunai, and then passed out due to a lack of chakra. Kakashi awakened to find their enemies defeated and Minato beside him. Minato explained that the special kunai he had given Kakashi had alerted him when it was thrown. Minato then apologized for not arriving sooner. The remaining team members cherished their memories of Obito, and went on to destroy the bridge.

From that point forward, Kakashi would become famous amongst all the five countries as, thanks to the gift Obito had given him. It was also revealed that, in the years that followed, Kakashi became more like Obito, losing track of time looking at Obito's name on the memorial and giving excuses that were similar to the ones Obito had given, as well as adopting Obito's belief that teamwork takes priority over following the rules. Kakashi visits the memorial stone located near the training grounds every morning in Obito's honor, making him late for almost everything.

Trivia

 * What Obito says to Kakashi when they argue about going after Rin is exactly what Kakashi will tell Squad 7 after they failed the bell test at first: "In the Ninja World, those who break the rules are trash, that's true, but those who abandon their friends are lower than trash."