A devastating zombie apocalypse has swept over San Francisco, leaving the city in ruins and overrun with hordes of the undead. The living are few, and among them are a small group of survivors who are struggling to fend off the relentless zombie onslaught. Among these survivors are a determined doctor, Lynn Snyder, who is desperate to find a cure for the virus that has turned humans into zombies, and a former baseball player, Marshall, who takes to the role of the group's protector with a fierce tenacity. The two of them, along with other survivors, take refuge on Alcatraz Island, believing its isolation provides a temporary safe haven from the zombie hordes.

The situation grows direr as it becomes apparent that Alcatraz is not the sanctuary they had hoped for. Supplies are running low, and the zombies are learning to swim, making the island increasingly vulnerable. In the face of these challenges, the group hatches a dangerous plan to escape to a supposed safe zone. They must now rely on each other and the leadership of Captain Caspian of the National Guard, a tough commander with a clear goal to keep his people alive. Preparations for their escape are hindered by the lurking threat of zombies and the emergence of internal conflicts that threaten to tear their fragile community apart.