In the aftermath of his fight with Kenpachi, Ichigo is left mortally wounded. Finding him, Yoruichi Shihōin transports him to a hidden chamber under Sōkyoku Hill, where she intends to keep him until he is healed. As she is explaining her flying device to him, they feel the Reiatsu of Captain Byakuya Kuchiki near the Senzaikyū, where Rukia Kuchiki is being held. Reminded of Ganju Shiba and Hanatarō Yamada, whom he sent to Rukia's rescue, Ichigo, snagging Yoruichi's device, leaves for the Senzaikyū. When he gets to the Senzaikyū bridge, he finds Ganju grievously injured by Byakuya's Shikai, and Hanatarō and Rukia pinned down by his Reiatsu. Confronting Byakuya, Ichigo challenges him. To everyone's amazement, Ichigo fights on an equal level with the captain, whose Zanpakutō is still sealed. Yoruichi arrives just in time to stop Byakuya from releasing his Zanpakutō against Ichigo.
Sedating Ichigo by inserting a tranquilizer into his open wound, Yoruichi escapes the pursuing Byakuya, along with Ichigo, by using Shunpo. She promises in three days, she will make Ichigo stronger than Byakuya. Yoruichi brings the anesthetized Ichigo back to Kisuke Urahara's secret training ground under the Sōkyoku Hill. When he wakes up, he is very mad at her for having rescued him and him alone out of all his allies on the bridge. He says he was the one with the best chance of defeating Byakuya, and because of her actions, Hanatarō, Ganju, and possibly Rukia might be dead. Yoruichi tells him he had no chance against Byakuya, and Captain Jūshirō Ukitake will not allow Byakuya to kill his allies or Rukia, who is his subordinate. Explaining how she could only carry one person and still hope to outrun Byakuya's Shunpo, she states she rescued Ichigo because she believes in three days, she can teach him a way to defeat Byakuya and rescue all his allies.
Lieutenant Renji Abarai awakens from his sleep in the prison cell, where he is recuperating from his injuries. His Zanpakutō, Zabimaru, materializing, urges him to get well soon so they can fight Ichigo and Zangetsu again. When Renji tells him he no longer considers Ichigo his enemy and has no quarrel with him, Zabimaru asks who Renji intends to fight. The next day, Renji escapes from his prison cell.
Day One, August 5th
Ichigo agrees to begin his training before he has fully healed from his injuries, though Yoruichi promises it will hurt quite a bit. As he, drawing his Zanpakutō, gets into a battle stance, Yoruichi, stopping him, asks if he had ever noticed his Zanpakutō was always in a released form. Saying he noticed it was larger than other Zanpakutō and would not change shape when he tried to release it, Ichigo recalls how Kenpachi's Zanpakutō was the same. Realizing he was unaware of it, Yoruichi, assuming he must have not known about the second level of release for a Zanpakutō, reveals the second release is called Bankai, and mastering it is one of the qualifications of becoming a captain of the Gotei 13. She tells Ichigo all the current captains besides one can perform the Bankai release. The one exception was Kenpachi, who is the only one in history to be made captain without even knowing the name of his Zanpakutō, but his raw strength and massive Reiryoku more than make up for this deficiency, which Ichigo should know full well. Yoruichi states while one's Bankai is dependent on the Zanpakutō's spirit and its wielder's strength and training, achieving Bankai can boost one's power by 5 to 10 times, which happens because achieving Bankai after attaining Shikai usually takes at least ten years of training. When Ichigo, protesting, says he does not have ten years to train, Yoruichi, calming him down, says she does not intend to use the proper method of training with him, but a much faster and far more dangerous method to get him to learn Bankai within three days.
At 11:33 AM, Yoruichi presents Ichigo with a strange looking doll. The doll's name is Tenshintai, an instrument developed by the Shinigami Research and Development Institute. The device is used to force a Zanpakutō spirit to materialize. She reveals in order to attain Shikai, communication is required between the wielder and the spirit, but in order to achieve Bankai, the materialization and submission of the spirit is required, which signifies the opposite of Shinigami going into the Zanpakutō spirit's world - instead, the spirit needs to be summoned into the Shinigami's world. Yoruichi says since Ichigo was able to stand up to Kenpachi to the end, he is already in the materialization level subconsciously. Ichigo remembers his fight with Kenpachi, where Zangetsu came to his aid after he was cut down, and Yoruichi realizes he has already experienced this. Telling Ichigo all he needs to do is stab the Tenshintai with his Zanpakutō and the spirit will materialize, after which she will keep it materialized with her power, Yoruichi tells Ichigo he only has one chance with it, and three days is the deadline to force Zangetsu into submission. Yoruichi intends to warn Ichigo further, but he, stabbing the instrument before she can continue, tells her he does not want to know because there is only one way to finish this, which is giving it all he has got until the end. Seconds later, as the Zanpakutō disappears from Ichigo's hand, the materialized Zangetsu, standing behind him, tells Ichigo he has healed up fast.
As Yoruichi asks Zangetsu if he heard her conversation with Ichigo, he states he did. Telling Zangetsu he will decide the nature of the training method, she asks if he can start right away. Agreeing, Zangetsu, crouching to the ground, touches it with one hand. This causes hundreds of swords, each resembling Zangetsu, but none exactly identical, to rise to the surface. Zangetsu says only one of the swords is the real Zangetsu, which is the only one capable of defeating him, and if Ichigo wants him to submit, he must find the real sword and use it to defeat Zangetsu before Zangetsu kills him. Each picking up a sword, Zangetsu and Ichigo attack. At 12:01 PM, Ichigo, exhausted, remains lying where he has fallen. As Yoruichi pushes him to get up, Zangetsu says if Ichigo cannot get up on his own, he will make Ichigo get to his feet by force.
When Zangetsu, breaking Ichigo's 18th sword, charges at him, Ichigo, spotting a sword similar in appearance to Zangetsu's familiar Shikai form, picks it up, believing he has found the true sword with which he will defeat Zangetsu. As the sword shatters the first time it meets Zangetsu's sword, Zangetsu's next attack connects, wounding Ichigo. Zangetsu reveals each of the swords represents a shard of Ichigo's soul: the true form of Zangetsu is the part of Ichigo which represents his will to fight, and the one which just shattered represented his dependence on Zangetsu, one of the most brittle parts of his soul. Stating if Ichigo cannot find strength in his own soul, he can forget about Bankai. he calls for Ichigo to get up and attack once more, for he does not have time to lie around. Getting up, Ichigo picks another sword.
After 51 swords, Yoruichi notices Ichigo no longer falls whenever one of his swords breaks. Pulling a jagged-bladed sword out of the hillside, Ichigo, coming at Zangetsu again, deflects Zangetsu's attack and responds with a jab of his own. The jab nearly hits its mark, but Zangetsu, leaping into the air, throws down his sword, breaking Ichigo's. Appearing next to Ichigo, he strikes him on the face, throwing him against a boulder. Telling Ichigo never to look away from his opponent, he tells him the last sword was his 52nd. Noticing a growth in Ichigo's fighting style, having deflected Zangetsu's attack instead of simply lunging forward, Yoruichi, contemplating the amazing speed at which Ichigo achieves his unbelievable advancement, thinks Kisuke might have been right about Ichigo from the start.
As the first training day draws to a close, Yoruichi, noticing each of Ichigo's swords lasts more than five minutes and it takes more than a normal impact to break them, realizes she is at her limits. As Ichigo is attacking with a scimitar-like sword, she ceases her maintenance of Zangetsu's materialization, causing him to revert into the Tenshintai. She declares to the perplexed Ichigo the first day of training is done. During his break, Ichigo, allowed a dip in the training ground's hot spring, wonders if the training day being over indicates it is night time, since there is perpetual light in the training grounds and he has lost his sense of time. As he washes his face, he realizes the water has healing properties, and all his aches and pains are gone. Putting water in his mouth, he, hoping it will heal the cut he has there, spews it out when Yoruichi states her intention of taking a bath as well. Relaxing when Yoruichi changes her shape into that of a cat, he asks Yoruichi about the training grounds, having noticed they look similar to the ones under the Urahara Shop.
She explains they were modeled after the ones under the Sōkyoku Hill, where Urahara built the grounds in secret when he and Yoruichi were young, and they used it to train there, sparring against each other to become stronger. As she states Urahara joined the Gotei 13 while she joined the Onmitsukidō, Ichigo, stopping Yoruichi's story, asks her who Urahara really is. Not surprised to find out Urahara did not tell Ichigo about his past, Yoruichi, saying there is no reason to hide it any longer, reveals Urahara is the former captain of the 12th Division, and the founder and former president of the SRDI.
Day Two, August 6th
Noticing a slow yet steady growth in Ichigo's power as his second day of training commences, Yoruichi cannot shake the feeling something bad is approaching. Her thoughts and Ichigo's training are brought to a halt by the crashing of a third party through the roof of the training grounds. The intruder turns out to be Renji, who, having figured out Ichigo was using the facility to hide and train for his Bankai, says he wants to join him, for they do not have much time. When Ichigo does not understand, Renji reveals Rukia's execution date has been rescheduled again - to the next day at noon. Renji admits in his current level, he cannot help Rukia, which is why he has gone to the training grounds to train for his own Bankai. He tells Ichigo not to worry, for he is already at materialization level and will not take long to achieve Bankai, so he will not be in the way of Ichigo's training. As he speaks, Renji's Zanpakutō spirit materializes. Crushed by the news, Yoruichi says there is no way Ichigo could possibly attain Bankai before the scheduled execution. When he hears this, Ichigo smashes the sword in his hand and tells Yoruichi since she was the one who suggested this training method, it is unbecoming she should be the first to give up. Ichigo states if the execution is scheduled for noon of the next day, he will just have to finish his training today.
Several hours later, having finished his training, Renji leaves the training grounds. Before departing, he asks Yoruichi if Ichigo will be successful. When she tells him to not worry, Renji says he did not mean he was worried for Ichigo, but was wondering if Ichigo is truly able to achieve Bankai. Yoruichi states she does not know, much to Renji's annoyance. Meanwhile, Zangetsu has raised the level of training by creating copies of himself, each holding a Zangetsu duplicate. Calling for Ichigo to come at him, he states he will not go easy on him. Ichigo tells his Zanpakutō spirit to not waste time on useless chatter, for he does not intend to give him a chance to go easy on him. Asking Renji if he remembers the day he learned to walk, Yoruichi states since no one remembers, but is still able to walk, walking must be an instinct. She says looking at Ichigo and seeing his determination, she thinks he knows instinctively that he can acquire the power. She believes he was born with the ability to learn Bankai.
As the time of the execution draws near, an exhausted Ichigo kneels with another one of his swords broken. When Zangetsu asks him if he is still able to stand up, Ichigo states he can because he has sworn to save Rukia. When Zangetsu asks to whom he has sworn, Ichigo states he swore an oath to his own soul.
Renji leaves the training ground to face up to his captain.
After making it to the execution grounds and stopping the execution, Ichigo consciously releases Bankai for the first time during his fight with Byakuya Kuchiki.