For a background and history refer to the bleach wiki.
The human world has recently been flooded by hundreds of hollow from Hueco Mundo. These hollow were headed by Grimjaw Jaggerjaques. The assault was placed at the time Soul Society declared an emergency lockdown. Soul Society, cutting off it’s own reinforcements, left the human world in the hands of the Shinigami currently stationed. Grimjaw’s hollow armies invaded three areas: Karakura Town, Berlin, Germany, and San Diego, California.