The entire Saga it's subdivided in 3 more sub-arcs, spacing in four years from 2006 to 2010.
The Arrancar Invasions
(volume 21-27) in the aftermatch of the Soul Society Arc, Ichigo and his friends go back to Earth, trying to live their lifes again. In the meantime, Aizen Sousuke got into Hueco Mundo, where he have already assembled an army to declare war to the Soul Society. As it is later revealed, his target it's the destruction of Karakura Town, to create the "King Key", that will allow him to attack the Spirit King of the Soul Society in his dimension. The "Invasions" are four different attacks from the Arrancar on Karakura and all the shinigami's forces in the city.
The First Invasion: it's a solitary raid from the imperfect arrancar Grand Fisher, who gets killed off by Ichigo father.
The Second Invasion: Ulquiorra and Yammy come to earth to "test" Ichigo powers, finding him "worthless trash".
The Third Invasion: unauthorized, Grimmjow take his gang and attack Karakura with the purpose of killing all possible enemies.
In one night of fights, five arrancars die, while Grimmjow is taken back in Hueco Mundo and scold for his failure from Aizen and Tousen. Finding himself too weak to the battles to come, Ichigo goes to the Vizards to be trained by them. While he's doing so, the Fourth Invasion starts: Grimmjow, Yammy, Luppi and Wonderwyce engages the shinigami contingent, while Ulquiorra kidnaps Orihime, bringing her to Hueco Mundo.
The Invasion of Hueco Mundo
(28-35) or simply "Hueco Mundo Arc". Ichigo, Ishida, Chad, Renji & Rukia attack Hueco Mundo and the Arrancar's fortress, Las Noches, with the intent of saving Orihime. In their struggle against the Espada they're later assisted by a task force of Soul Society's Captains. Main conflicts during the invasion:
The Privaron Espada's Fights
The Battle against Aaroniero Arruruerie: Rukia defeats the 9th Espada after he tried to trick her disguising himself as a long lost friend of the shinigami.
Ichigo vs Grimmjow
The Battle against Zommari LeRoux: the 7th Espada against Byakuya Kuchiki.
The Battle against Szayel Aporro Grantz: one of the longest and most elaborated fights of this Arc, that involved seven different characters.
The Battle against Nnoitra Jiruga: Zaraki, Ichigo and Nelliel vs the 5th Espada. At the end of the confrontation with Jiruga, Aizen set his plan in motion, trapping the Shinigami in the Hueco Mundo, while his main forces attack Earth.
The Battle against Ulquiorra Schiffer: Ichigo vs the 4th Espada. Takes place during the Winter War.
The Battle against Yammy Ryalgo: Zaraki, Byakuya & Mayuri vs the Espada Zero. Takes place during the Winter War and the Deicide Arc.
Turn Back The Pendulum
(36) or "The Pendulum Arc" the shortest arc, that doesn't even features the Arrancars. In a pause from the actual story, we explore the Soul Society of 100 years in the past, discovering that the Vizards are all ex-members of the Gotei 13, victims of Aizen, who's beginning to create his plan.
The Winter War
(37-45) also known as "Fake-Karakura Town Arc", the forces of Gotei 13 gather around a spiritual version of Karakura Town, engaging Aizen, Gin, Tousen and the Top Three Espada with their foot soldiers. The Battle Consist of four phases:
The Battle at the Four Pillars
The Fracciones Battles
The Espada Fights
Gotei & Wizards vs Aizen
The Deicide Arc
(46-48) the second shortest tale of the Saga, and also the second one that pretty much doesn't feature any Arrancar at all (except in the ending). After single handedly defeat the entire Gotei 13, Aizen clash against Ichigo Kurosaki, and later against the newcomers Urahara, Yoruichi and Isshin (while Ichigo confronts Gin). All this fights, activates the Hogyoku that Aizen have placed in his chest, beginning the process that will allow him to become a God.
During this transformations, Aizen defeats his enemies and along with Gin goes to the Soul Society to finally create the King Key. Here, Gin betrays his master, trying to kill him. The only thing he manage to do it's forcing even more the mutation process, making Aizen stronger.
But the final battle of the Saga finally begin: Ichigo, with the help of his father, have sustained a "spiritual training" inside his own mind, completly dominating his zanpakuto. When he clashes with Aizen, his able to overpower him with the god-like "Mungetsu". But even this is not enough. Aizen resurface from the battlefield while Ichigo loses all his powers in echange for the weapon previously used.
It's Urahara who come and save the day: during his previous fight he have placed a hidden seal in his attacks that would activet himself in the moment of major weakness of the New God. Aizen it's finally defeated, sentenced to 38.800 years of prison (because he's now immortal) in the deepest hole of the Soul Society, while the natural order it's restored. Unfortunately Ichigo have lost all his powers, making him impossible to even communicate with all the others shinigami or spirits.
The Arrancar Saga ends with him giving his farewell to Rukia and the Soul Society.
Hell Arc Prologue
The prologue to the "Hell Arc" movie is linked to the events of the saga: it show us the destiny of Apollo Grantz e Aaroniero, two of the major villains during the Hueco Mundo Invasion. Both of them end up in hell after their defeat.