The actress told Collider that suave news anchor Hector (Dominic West) "falls apart at the seams" in the BBC drama's second run.
"There are some significant changes," Garai - who plays Bel Rowley - explained. "We have a new Head of News, played by Peter Capaldi. He's a big new character.
"The first season of The Hour concentrated on the Suez Crisis, which was a relic of the empire conflict. This season is much more about the Cold War."
Garai added that the "two main threads" of the newsroom thriller's next series will be "celebrity and the London underworld of the time" and "the arms race between America and Russia".
"Those two themes combine in the story," she revealed. "Freddie (Ben Whishaw) has a new love interest. [My character] Bel has a new love interest. Hector (Dominic West) falls apart at the seams. There's a lot of change."
Hannah Tointon (The Inbetweeners) and Tom Burke (State of Play) will also join The Hour for its second series.
The show will return to BBC Two later this year.
> The Wire star Dominic West admits to writing on TV internet forums
> The Hour star Romola Garai confirms series two details
Watch a BBC Two drama trailer - featuring clips from The Hour series two - below: