In 1971, during the Vietnam War, the US military sprayed massive amounts of a new defoliant to cut off Viet Cong supplies. However, the new defoliant caused a mutation, turning the Viet Cong and South Vietnamese residents who were exposed into zombies that attacked the US and Allied forces. Upon learning this, the US immediately launched large-scale bombardments, but the damage to their own troops only worsened. Eventually, as US and Allied bases were overrun by zombies, they deployed napalm. While effective at first, the infected still managed to reach North Vietnam. Realizing the severity, the United Nations blockaded both South and North Vietnam. The US issued an evacuation order for its remaining troops, but by the time the withdrawal was complete, some US and Allied soldiers were left behind. Thomas, a Sergeant in the 1st Infantry Division's jungle unit, missed the last evacuation helicopter. While he stood in despair, infected Viet Cong and Allied soldiers began approaching. Thomas fled into the jungle to survive. The things he feared most—Viet Cong and traps—were replaced by the fear of infected popping out at any moment. He used his M16 to take them down, but their numbers only grew. Forced to run blindly, he reached a cliff. With a drop behind him and the infected closing in, Thomas jumped to save his life. When he woke up, an Allied soldier had found him and was treating him in a hideout. The soldier was Sergeant Kang Sanho of the Korean White Horse Division, a young man who volunteered for the war to support his sick mother and family. Like Thomas, Sanho had failed to escape. After a brief introduction, Thomas noticed a civilian guarding the door. It was Rasha, a former Viet Cong. He had fought the Americans to communize Vietnam, but now he was fighting the infected and living in the hideout with Sanho. Thomas wasn't pleased, but he needed to escape. He remembered the communication equipment at his former base, but getting there meant going back into the jungle. Staying put wasn't an option, so Thomas proposed they escape together. The three of them headed back into the dangerous jungle, dodging traps, looters, and the infected.