Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Jenna Coleman (Clara Oswald)
Colin McFarlane – Moran
Sophie Stone – Cass
Zaqi Ismail – Lunn
Morven Christie – O'Donnell
Arsher Ali – Bennett
Steven Robertson – Pritchard
Paul Kaye – Prentis
Directed by Daniel O'Hara
Written by Toby Whithouse
Script editor Nick Lambon
Produced by Derek Ritchie
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 9
Length 1st of 2-part story, 45 minutes
Originally broadcast 3 October 2015
The crew of an underwater base on Earth in 2119 discover an alien craft in the ruins of a submerged town. They bring it aboard only to find it is missing one of its two power cells, and four glyphs carved into the interior walls. Suddenly the craft's engines ignite, and Commander Moran, is killed in the act of pushing another out of the way. Later, the crew finds two ghostly figures, Moran and an unidentified alien, roaming during the night cycle, soundlessly chanting before turning to attack the crew. The crew take shelter in the base's Faraday cage which the ghosts cannot enter.
The Doctor and Clara arrive later via the TARDIS and are briefed by acting commander Cass, who is deaf and assisted by her translator Lunn. Suddenly the base's systems switch to the night cycle, and before they can restore the day cycle, the ghosts have killed another crew member, and soon the ghost of that crew joins the other two. The Doctor recognises the alien ghost as Tivolian, a cowardly, peaceful race. Cass calls for an evacuation shuttle only to learn one has already been dispatched. The Doctor orders it to turn around, believing the ghosts have a nefarious purpose.
The Doctor is able to lure the ghosts into the Faraday cage, and with Cass's help, can read what they are chanting: "the dark, the sword, the forsaken, the temple". The Doctor determines these are galactic coordinates pointing to this location, and believes that the ghosts are killing the crew and trying to draw more victims to boost the strength of the broadcast of this signal; however, only those that have seen the glyphs in the ship are susceptible to the ghosts' influence. Using the base's remote drones, they find and collect a stasis chamber among the ruins, which the Doctor suspects is what the coordinates point to, but they cannot open it. The Doctor determines he must go back in time before the town was flooded to find the origin of the signal. The ghosts manipulate hatchways that separate Clara, Cass, and Lunn from the Doctor and crew members O'Donnell and Bennett. The Doctor promises Clara he will be back, and takes O'Donnell and Bennett with him to the TARDIS. Shortly after the TARDIS' departure, Clara, Cass, and Lunn witness a new ghost outside of the base - that of the Doctor.