Before the Flood

Jack, Sydney, Irina, Vaughn and Nadia parachute into Sovogda, Russa to dismantle the giant floating red ball of the Circumfrence which is turning people into mindless, red-eyed zombies out to kill. Back at APO, Marshall and Weiss try use the Blackwell Index to blackmail the Russian defense minister into giving them the access codes to a Russian satellite before the local authorities plan to drop a nuclear bomb on the city to destroy it and prevent the spread of the contagion, and they also discover that Elena is up linking the satellite. In the ruins of the city, a CIA agent, Brodien (the one whom Sydney rescued from Shanghai in the season opener) reminds them that the Rambaldi device is broadcasting a sub audible frequency that physiologically alters people who drink the water-like fluid that has infected the city's water supply and it is irreversible. At the same time, Elena learns of the APO team's arrival and sends Sloane and her men to go out to kill them, leading to a three-way war between Elena's henchman, the APO group, and the zombies leading to Nadia becoming infecting and having a final one-on-one confrontation with Sydney atop a building with the red ball Circumference in the balance.