The crew of an underwater mining facility in Caithness, Scotland, discovers an alien spaceship in the ruins of a submerged town. They bring it aboard. One of its power cells is missing, there are mysterious glyphs, and it kills Commander Moran (Colin McFarlane). Then, two ghostly figures appear, Moran and an unidentified alien, wandering in the night and silently chanting. They attack the crew, who shelter in a Faraday Cage.

The Doctor (Peter Capaldi) and Clara (Jenna Coleman) arrive in the TARDIS and are briefed by Commander Cass (Sophie Stone), who is deaf, and interpreted by Lunn (Zaqi Ismail). The ghosts kill another crewmember, who becomes a ghost. The Doctor recognizes the ghosts as Tivolians, a harmless and peaceful race. Cass calls for a rescue shuttle and finds there is already one on the way. But the Doctor orders it away, so the ghosts cannot escape on it.

The Doctor lures the ghosts into the Faraday Cage, and Cass reads their lips. They are chanting, “The dark, the sword, the forsaken, the temple.” The Doctor knows these are galactic coordinates. Only the crew who have seen the glyphs are susceptible. They find a stasis chamber in the ruins, which the Doctor believes is the location of the coordinates, but they can’t open it.

The Doctor must go back in time to before the town was flooded. The ghosts manipulate hatches and separate Cass and Dunn from the Doctor, O’Donnell (Morven Christie) and Bennett (Arsher Ali). The Doctor promises Clara he will be back and takes O’Donnell and Bennett on the TARDIS. Shortly thereafter, Clara and the others see a new ghost outside—The Doctor. To be continued.

The story was well-reviewed and called creepy and atmospheric. The cliff-hanger ending was called brilliant. Clara has cue-cards for the Doctor to read, one of which reads, “No-one is going to get eaten, vaporised, possessed, exterminated, upgraded, mortally wounded, or turned to jelly, and we’ll all get out of this unharmed.” Another reads, “I didn’t mean to imply that I don’t care.” He tries to sign “Go ahead” to Cass, but signs “You are beautiful,” and is puzzled by her reaction.