A fourth series of the Doctor Who spinoff is currently in production as part of a deal between BBC Worldwide, BBC Wales and US network Starz.
Davies told SFX: "It’s too soon to give away the story, but I’ve always had this story in mind, and the whole existence of season four will make sense once you know what the story is."
The writer compared the tone of the new episodes to that of previous miniseries Torchwood: Children of Earth.
"This story is also very dark," he revealed. "I think with [Children of Earth], Torchwood found its feet."
He promised that the fourth series will not disappoint dedicated fans, but will also appeal to new viewers.
“For faithful viewers you’ll see the same people in the same world, following on," he said. "Though it also starts from scratch, because there’s a brand new Starz audience there... waiting to be scooped up. But I restarted Doctor Who in 2005 from scratch, so I know what I’m doing."
He added: "It’s still Gwen, it’s still Jack, and hopefully more. You’ll feel very comfortable with it. And there’s great new American characters coming into it."
Torchwood is expected to air on Starz in the US and on BBC One in the UK next summer.