Bankai Training is an event that follows Ichigo's fight with Kenpachi Zaraki and before Rukia's execution.
In the aftermath of his fight with Kenpachi Zaraki, Ichigo is left mortally wounded. Yoruichi finds him and transports him to a hidden chamber under Sōkyoku Hill, where she intends to keep him till he is healed. However, as she is explaining to him the nature of her flying device, they suddenly feel the Reiryoku of Byakuya Kuchiki near the Senzaikyū, where Rukia was being held. Ichigo is reminded of Ganju and Hanatarō, whom he sent to Rukia's rescue. He snags Yoruichi's device and leaves for the Senzaikyū. When he gets to the Senzaikyū bridge he finds Ganju already grievously injured by Byakuya's Shikai and Hanatarō and Rukia pinned down by his Reiatsu. Ichigo confronts Byakuya and challenges him. To everyone's amazement, Ichigo fights on an equal level with the captain in his pre-release. However, Yoruichi arrives just in time to stop Byakuya from using his release against Ichigo.
She sedates Ichigo by inserting a tranquilizer into his open wound and then manages to escape the pursuing Byakuya, along with Ichigo, using Shunpo. She promises that in three days time she will bring Ichigo back, 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 cross with 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 that he had no chance against Byakuya, and the presence of Jūshirō Ukitake on the bridge won't allow Byakuya to kill his allies or Rukia, who is his subordinate. She also explains that she could only carry one person and still hope to outrun Byakuya's Shunpo. She rescued Ichigo because she believes in three days time she could teach him a way to defeat Byakuya and rescue all his allies. 
Renji awakens from his sleep in the prison cell where he is recuperating from his injuries. His Zanpakutō, Zabimaru materializes and urges him to get well soon, so that they can fight Ichigo and Zangetsu again. Renji tells him that he no longer considers Ichigo his enemy, and has no quarrel with him. Zabimaru then asks who Renji intends to fight. The next day Renji escapes his prison cell and disappears. 
Day One, August 5th
Ichigo agrees to begin his training before he has fully healed from his injuries, although Yoruichi promises it would hurt quite a bit. He draws his Zanpakutō and gets into battle stance, but Yoruichi stops him and asks if he had ever noticed his Zanpakutō was always in a released form. Ichigo says he did notice it was larger than other Zanpakutō, and wouldn't change shape when he tried to release it. He remembered that Zaraki's Zanpakutō was the same. Yoruichi realizes he was unaware of it and assumes he must have not known about the second level release for a Zanpakutō. She explains that the second release is called Bankai, and that 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 being Zaraki. The 11th Division captain 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, as Ichigo should know full well. Yoruichi explains that while one's Bankai is dependent on the Zanpakutō's spirit and its wielder's strength and training, achieving a Bankai can boost one's power between 5 to 10 times. This is due to the fact that achieving the Bankai after attaining Shikai usually takes at least ten years of training. When Ichigo protests, saying he doesn't have ten years to train, Yoruichi calms him down and says she doesn't intend to use the proper method of training with him, but a much faster yet 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 R&D Institute, which is used to force a Zanpakutō spirit to materialize. She explains that in order to attain Shikai, communication is required between the wielder and the spirit, but in order to achieve Bankai what is required is the materialization and submission of the spirit. This signifies the opposite of the Shinigami going into the Zanpakutō spirit's world; instead the spirit needs to be summoned into the Shinigami's world. Yoruichi says that, since Ichigo was able to stand up to Zaraki to the end, he's 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 had already experienced that. All Ichigo needs to do is stab the Tenshintai with his Zanpakutō and the spirit will materialize, after which Yoruichi will keep it materialized with her power. She warns Ichigo that he only has one chance with it and that three days is the final deadline to force Zangetsu into submission. Yoruichi intends to warn Ichigo further, but he stabs the instrument before she can continue, telling her he doesn't want to know. There is only one way to finish this and that is giving it all he's got, to the end. Seconds later, the Zanpakutō disappears from Ichigo's hand and the materialized Zangetsu stands behind him. He tells Ichigo that he'd healed up fast.
Yoruichi asks Zangetsu if he had heard her conversation with Ichigo, he replies the he has. Yoruichi tells Zangetsu he will decide the nature of the training method, she then asks if he can start right away, Zangetsu agrees and crouches to the ground, touching 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 that can defeat him. If Ichigo wants him to submit, he must find the real sword before Zangetsu kills him and then use it to defeat Zangetsu. Zangetsu and Ichigo each pick up a sword and attack. At 12:01 PM, Ichigo is already exhausted and remains lying where he's fallen. Yoruichi pushes him to get up, but Zangetsu says if Ichigo can't get up on his own, then he will make Ichigo get to his feet by force.
When Zangetsu breaks Ichigo's 18th sword and charges at him, Ichigo spots a sword similar in appearance to Zangetsu's familiar Shikai form. He picks it up believing he had found the true sword with which he will defeat Zangetsu, however, the sword shatters the first time it meets Zangetsu's sword. Zangetsu's next attack connects, wounding Ichigo. He explains that each of the swords represents a shard of Ichigo's soul, the true form of Zangetsu is the part of Ichigo that represents his will to fight, and the one that just shattered represented his dependence on Zangetsu, one of the most brittle parts of his soul. 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, as he doesn't have time to lie around. Ichigo gets up and picks another sword.
After 51 swords Yoruichi notices that Ichigo no longer falls whenever one of his swords breaks. Ichigo pulls a jagged bladed sword out of the hillside and comes at Zangetsu again. He is able to deflect Zangetsu's attack and respond with a jab of his own. The jab nearly hits its mark, but Zangetsu leaps into the air and throws down his sword, breaking Ichigo's. He then appears next to Ichigo and strikes him on the face, throwing him against a boulder. He tells Ichigo never to look away from his opponent and informs him that the last sword was his 52nd. Yoruichi thinks she notices a growth in Ichigo's fighting style, deflecting Zangetsu's attack instead of simply lunging forward. She contemplates about the amazing speed in which Ichigo achieves his unbelievable advancement and thinks Urahara might have been right about Ichigo from the start.
As the first training day draws to a close, Yoruichi notices that each of Ichigo's swords lasts more than five minutes and that it takes more than normal impact to break them. However, she realizes she's at her limits, and as Ichigo is attacking with a scimitar like sword, she ceases her maintenance of Zangetsu's materialization, causing him to revert back into the Tenshintai. She declares to the perplexed Ichigo that the first day of training is done. During his break, Ichigo is allowed a dip in the training ground's hot spring. He wonders if the fact that the training day is over indicates that 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 that all his aches and pains are gone. He puts water in his mouth hoping it will heal the cut he has there, but soon spews it out when Yoruichi states her intention of taking a bath as well. He only relaxes 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 that those were modeled after the ones under the Sōkyoku hill. 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. Urahara joined the Gotei 13 while she joined the Onmitsukidō. Ichigo stops Yoruichi's story to ask her who Urahara really is. Yoruichi isn't surprised to find out Urahara didn't tell Ichigo about his past, but says there isn't a reason to hide it any longer. Urahara was the former captain of the 12th Division and the founder and former president of the Shinigami Research and Development Institute.
Day Two, August 6th
Yoruichi notices a slow yet steady growth in Ichigo's power as his second day of training commences. However, she cannot shake the feeling that something bad is approaching. Her thoughts and Ichigo's training are brought to a halt by the crushing of a third party through the roof of the training grounds. The intruder turns out to be Renji Abarai. He figured out that Ichigo was using the facility to hide and train for his Bankai and says he wants to join him, as they don't have much time. Ichigo doesn't understand and Renji explains that Rukia's execution date has been rescheduled again - to the next day at noon. Renji admits that in his current level he can't help Rukia, which is why he has gone to the training grounds, to train for his own Bankai. He tells Ichigo not to worry, as he is already at materialization level and won't take long to achieve Bankai, so he won't be in the way of Ichigo's training. As he speaks, Renji's Zanpakutō spirit materializes. Yoruichi is crushed by the news. She says there is no way Ichigo could possibly reach Bankai before the scheduled execution. When he hears this, Ichigo smashes the sword in his hand and tells Yoruichi that since she was the one who suggested this training method, it is unbecoming that she should be the first to give up. Yoruichi asks what will happen if Ichigo doesn't succeed in time, but Ichigo replies that if the execution was scheduled for noon of the next day, then he'll just have to finish his training by then.
Several hours later, having finished his training, Renji leaves the Training grounds. Before departing he asks Yoruichi if Ichigo will be successful. She tells him not to worry, but Renji says he didn't mean he was worried for Ichigo, but wondering if Ichigo was truly able to achieve Bankai. Yoruichi replies that she doesn't know, much to Renji's annoyance. In the meanwhile, Zangetsu has raised the level of training by creating copies of himself, each holding a Zangetsu duplicate. He calls for Ichigo to come at him and warns him he won't go easy on him. Ichigo tells his Zanpakutō spirit not to waste time on useless chatter, as he doesn't intend to give him a chance to go easy on him. Yoruichi asks Renji if he remembers the day he learned to walk and adds that since no one remembers, but are still able to walk, then 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, Ichigo finds himself exhausted and kneeling with another one of his swords broken. Zangetsu asks him if he is still able to stand up and Ichigo replies that he can, because he's sworn to save Rukia. Zangetsu asks to whom he has sworn. Ichigo replies that he swore an oath to his own soul.
Renji leaves the training ground to face up to his captain.
Using his newly acquired Bankai, Ichigo makes it to the execution in time to stop it. Later, he consciously releases the Bankai for the first time during his fight with Byakuya Kuchiki.