The Skyfall actor reprises his role as reporter Freddie Lyon for the BBC Two drama, which delves behind the scenes of a current affairs programme in the 1950s.
Speaking to Digital Spy and other journalists, Whishaw praised writer Abi Morgan for doing an "amazing job" of taking The Hour to unsettling new places.
"I'm really blown away," he admitted. "I think it reaches a really brilliant climax. It's quite shocking and quite dark. It goes into a slightly darker place, maybe than it's gone before."
On the themes that will be explored in The Hour, Whishaw continued: "There's a lot of ingredients. I suppose the main story is really about Soho and this underworld of crime, and again how that is connected to people in positions of power.
"The nuclear race is a big thing and Abi, again very cleverly, ties those two together. But in the background there's also a sort of race issue and it touches briefly on something called the Wolfenden Report, which was a report into whether homosexuality should be legalised. So there's a sexual morality theme as well."
Whishaw added that Freddie will return a "quite different" character following his firing from the programme at the end of series one.
"[He's] maybe a little better at playing the game or something, because he shoots himself in the foot quite a lot in the first series," he said.
"So [he's] maybe a bit wilier, a bit more political, a bit more understanding of the politics of a situation before he just charges in.
"He's sort of driven by Bel (Romola Garai). [She] has an almost kind of feminist concern with the girls who work at the Soho clubs, so she's really driving the story at first and Freddie kind of is joining her story, so he's sort of working more in collaboration with her".
Whishaw also said that he would be disappointed if The Hour is not given the chance for a proper send-off.
"I really loved the character," he stated. "I never really had played anyone quite like Freddie, so it was a very easy decision and I really hope that we get to do a third one and finish the story off.
"I think it would be sad if we didn't. But that's out of my control. I think that the BBC want to do a third one."
The Hour, which also stars Dominic West, Anna Chancellor and Julian Rhind-Tutt, returns on Wednesday (November 14) at 9pm on BBC Two in the UK.
The show will return in the US on Wednesday, November 28, airing on BBC America.