The game is set during a zombie outbreak in Kamurocho, a big departure from the storyline of the series to this point. The game is set during April 2011, when the residents of Kamurocho have become zombies. Witnessing the start of the outbreak through the eyes of Shun Akiyama, a local loan shark. Kamurocho is slowly quarintined as the Japan Ground Self-Defense Force is called in to contain the outbreak. Now only one soldier remains alive, Misuzu Asagi. After receiving a phone call from a mysterious man who has kidnapped Haruka, Kiryu returns from Okinawa. Now, four men are the only hope - the mysterious money-lender Shun Akiyama, feared yakuza and construction company owner "Mad Dog" Goro Majima, the Dragon of Kansai who has finally awoken from a long sleep, Ryuji Goda, and the legendary former yakuza, Kazuma Kiryu. These four men are the only people who can defend the town, and must fight to save it.
Source: Wikipedia, "Yakuza: Dead Souls", available under the CC-BY-SA License.