MUSICAL BIOHAZARD ~ Voice of Gaia ~ is set in an unspecified year where the threat of bioterrorism has continued to increase in extremity. Much of the story is set in and around a walled Croatian town on the Adriatic Sea, which faces attack from "Them", mutant humans infected with a virus that drives them to perform animalistic acts, who so far have failed to breach the wall.
The story focuses on Lisa Martin (Reon Yuzuki), who awaits the return of a gathering team sent out to collect food for the city. The team suffers heavy casualties and its survivors are infected; they bring with them a girl who's immune system is resistant to the viral infection. The city's physician, Dan Gibson (Daisuke Watanabe) begins formulating a serum from the girl's antibodies. Meanwhile, he and Lovro (Soichi Hirama) work with US Air Force Colonel Maurice Green (Arikawa Makoto) in investigating the small island the girl came from. Meanwhile, to keep everyone's minds off the compromised city defenses and the deteriorating condition of the gathering team, the city holds a music festival featuring the musician Roberto (Shigehito Naoto/Murai Kaiho).