The movie opens with a recap of Ichigo Kurosaki's final battle with Ulquiorra in Hueco Mundo, which Ichigo wins after being forced to transform into a incredibly powerful and completely uncontrollable Hollow. Some time later (i.e. after the completion of the Arrancar Downfall Arc (which also includes the war in Hueco Mundo, The Fake Karakura Town Arc, and The Deicide Arc)); Ichigo is living peacefully in the real world when Rukia Kuchiki and Renji Abarai arrive, but refuse to inform Ichigo of the reason for their visit. Shortly after this, Ichigo's school is attacked by a set of powerful, mask-wearing spirits called the Unforgiven (Togabito). When one of the Unforgiven has his mask broken it is revealed that they have escaped from Hell: breaking the mask triggers the appearance of one of the guardians of Hell, a Kushanāda, who then impales the unmasked Unforgiven and drags him back to Hell.
However, the battle at the school turns out to be a ruse; while Ichigo and his friends are engaged in battle, Ichigo's two sisters are attacked by Shuren, the leader of the Unforgiven. Ichigo manages to return in time to attack Shuren, but is unable to defeat him. An Unforgiven named Kokutō rescues Karin Kurosaki, but Shuren manages to depart with Ichigo's younger sister, Yuzu Kurosaki. Kokutō offers to assist Ichigo by showing him the route into Hell, and Rukia, Renji, and Uryū Ishida decide to join the quest. During the journey through the upper levels of Hell, the group is attacked by Kushanāda, and during the battle Ichigo's Hollow mask begins to spontaneously manifest. Kokutō warns him to avoid using his mask lest he lose control and hollowfy entirely. It is also revealed that spirits killed in Hell become Unforgiven themselves, and that the Kushanāda torture Unforgiven for their sins by catching and consuming them; consumed spirits are eventually reborn at a lower level of Hell, after which they are again caught and consumed by the Kushanāda in a cycle which continues until the spirit is completely crushed.
The group then catches up to Shuren's allies and engage in battle while Ichigo and Kokutō move on and locate Shuren's lair, where Yuzu is locked in a cage. Shuren reveals that he wants Ichigo to use his Hollow powers to destroy the Gates of Hell in the belief that this will free the Unforgiven from Hell. Once Shuren and the other Unforgiven are defeated, Kokutō betrays and attacks Ichigo, revealing he was the one behind Yuzu's kidnapping, as he also wishes to use Ichigo's power, believing that it can be used to break the invisible chains which tie him to Hell. Kokutō then taunts Ichigo by revealing that Yuzu has become an Unforgiven, after which he cuts down Rukia, Ishida, and Renji. An enraged Ichigo transforms into his Hollow form and blasts Kokutō with his Cero, destroying most of the chains binding him to Hell and burning through the gates of Hell. Renji breaks Ichigo's Hollow mask and activates a kido that teleports Ichigo and Yuzu out of Hell, leaving himself, Rukia, and Ishida trapped in Hell.
After Orihime fails to heal Yuzu, Ichigo is put under surveillance by Soul Society to prevent him from returning to Hell, as the damage caused by the battle has spilled into the real world. However, when Yuzu spontaneously recovers, Ichigo decides to return to Hell and rescue his friends. Returning to Shuren's lair, he finds Kokutō, who reveals that Rukia has turned into an Unforgiven while Renji and Ishida have been killed. Ichigo fights Kokutō and attempts to resist transforming into his Hollow form, which attracts the attention of the Kushanāda. The Kushanāda grant him the power to defeat and seal Kokutō away in the depths of hell, as well as the ability to free his friends from the Unforgiven chains. The group then flees Hell and emerge back in the real world as the gates repair themselves and vanish.