Rewind to the eve of the apocalypse, where a conflicted officer and new father (Nick Stahl) fights for survival just as the apocalypse hits. Shipmates turn one-by-one with no explanation, and the USS Pennsylvania becomes a nuclear-fueled and walker-filled death trap with no way out. After a crewmember suffering from appendicitis dies on board despite the medics' attempts to save him, he eventually turns and manages to bite several people. From there, things spiral out of control on the submarine as an outbreak quickly spreads, killing the majority of the crew within a day. Ultimately several surviving crewmembers were able to raise the submarine to the surface, allowing for eight people, including Riley, McGuire, and Walter, to escape from the submarine with their lives.