In 2000, a U.S. helicopter is shot down in the country of Penamstan; its crew is attacked by the local people. Despite their contrary orders, the Mad Dogs unit lands and tries to save the crew. Six years later, Claire works as an aid worker in Penamstan, where she discovers hints that the survivors of the ongoing conflict were exposed to bio-organic weapons (B.O.W.s). Afterwards, the White House is attacked by a hacking incident, followed by several employees being infected. With hints pointing
Leon, Jason, and Shen May head in their submarine into Chinese waters while Claire researches into what happened to the Mad Dogs. Their mission is also shown during a flashback in which the Mad Dogs rescued a survivor, but could only bring back his infected body. While on the submarine, rat-like B.O.W.s attack and kill the crew. Leon, Jason and Shen May evacuate the submarine together in an escape pod before it self-destructs. The destruction is blamed on a nearby Chinese fleet by Defense Secret
Claire investigates the house of the last survivor of the Mad Dogs besides Jason, but he has already killed himself. It is revealed that the unit's soldiers were infected during their mission in Penamstan. Meanwhile, Leon follows Shen May to her grandfather's home. After he forces his way in, it is revealed that Shen May's brother was the soldier that was saved by the Mad Dogs, but he was already infected with the virus and turned when the Mad Dogs brought him back to base. Shen May's grandfathe
Claire awakes in a laboratory after being captured by Wilson's men, while President Graham prepares a peace accord with the government of Penamstan. Wilson reveals that he is breeding supersoldiers to use them against the enemies of the United States. Wilson then tries to convince Claire to cooperate with him, but she refuses. Jason reappears and turns into a B.O.W. as Leon and Shen May arrive. Jason infects Wilson and fights with Leon and Shen May, as he tries to reveal himself to the world. Le