Matt Smith

After last week's outer space journey, Doctor Who is literally diving into its next adventure... underwater!

"[Clara and the Doctor] (Matt Smith) turn up on this Russian submarine that is effectively crashing," Jenna-Louise Coleman, who plays companion Clara, tells TVGuide.com. "They have the Ice Warrior, an alien creature, here on board and they're in the midst of the Cold War, so you don't know who's going to fire on who."

Jenna-Louise Coleman gives the lowdown on Doctor Who's new companion

An alien, the Doctor, Clara and a Navy crew all stuck in a submarine? As one might expect, the set was fairly cramped. "It was kind of made to size, but obviously we had the Ice Warrior on set, so he looks huge amongst all of it," she says. "We had to get through the big portholes and that kind of thing. It was brilliant."

Coleman also ran into another unique condition on set: acting while wet. "It was supposed to be an old submarine and we were really far under the water, so everywhere it was dripping," she explains. "We were wet for the entire episode. And we did it properly, so we had drips from the ceiling constantly. We were up to ankles in the water for the whole two weeks on set. So you go into makeup and have a makedown, like a bucket of water on your head in the morning."

Despite the physical challenges and action sequences, Coleman promises that "it does have quite an emotional twist at the end."

Check out this preview of "Cold War." Doctor Who airs on Saturdays at 8/7c on BBC America.