The threat to the United States in Season 5's white-knuckle day is Russian separatists armed with weaponized nerve gas and led by Vladimir Bierko (Julian Sands). Inciting their ire is an arms and mutual defense treaty that Russian president Yuri Suvarov (Nick Jameson) plans on signing with the U.S. and duplicitous president Charles Logan (Gregory Itzin). As Day 5 begins, Jack Bauer (Kiefer Sutherland), who faked his own demise at the close of last season, is working at an oil refinery in California under the name Frank Flynn. He returns from his self-imposed exile to fight the good fight after an assassination rocks the nation and he finds himself framed for it and several other crimes. Jack is also reunited with former love interest Audrey Raines (Kim Raver), but tumult continues to follow them. Meanwhile at CTU, there's a new man brought in to oversee operations, Lynn McGill (Sean Astin), and once again, there's a mole in the ranks. A significant subplot centers on Christopher Henderson (Peter Weller), Jack's mentor-turned-nemesis who has information that can help Jack thwart the Russians. Another thread follows First Lady Martha Logan (Jean Smart), who grows increasingly disdainful of her husband's actions in office.
by Fred Mitchell synopsis