A biochemical virus sweeps through the city. Mike, a gold-medal personal trainer, and Steve, who seems weak and timid, are trapped in a top-floor gym. Mike is bitten by a high-level mutant while shielding Steve. As he lies dying, he confesses his love for Steve and begs him to end his life. Steve then tears off his disguise, revealing his true identity: the military's Zero Immune Carrier and a retired special forces assassin. To retrieve the original serum from the ground floor and save Mike, he ventures alone through 50 floors.
