In the end, Soul Society defended Karakura Town by means of a "fake" town. Obviously, Sosuke Aizen was never fooled by this. He played along. But just by doing so, the end of Hueco Mundo came. The captains and the rest of Soul Society wiped out the Espada, Exequias, Numero, and Fraccion, leaving only three of the current Espada to survive. Although it meant that Aizen retreated, it didn't mean all of the Gotei Thirteen made it out safe. In the end of that battle, Komamura Sajin never made it out alive. But before he died, he knew that it was for the Sotaicho and he was fine with it.

But now the current problem is that there are four captain positions open, and they belonged to Gin Ichimaru, Sosuke Aizen, Komamura Sajin, and Kaname Tosen. Several shinigami trained their hardest, in hopes of achieving bankai, but during this time, it was that the traitors remained quiet. But the most quiet of all, was Ichimaru Gin. At one point, even Aizen didn't know where he had wandered off to, but then he knew. The fox-like man went deep into the desert of Hueco Mundo, and looked for an underground cavern. When he had finally found it, he "awoke" four souls. One was a shinigami, one was an arrancar, another was a quincy, and the fourth, well, we honestly never figured that out.

View extended plot here