Introductions

In an underwater base in the year 2119, a joint military-industrial team discover an alien vessel among the ruins of a long-submerged town. While examining it, they uncover mysterious symbols carved on the inside of the stark white interior. When one of the vessel's engines activates, the hangar catches on fire. The group flees the room but their commanding officer, Moran, is killed when pushing another out of the way of the exhaust blast. Once they have left the hangar, the group head to a corridor, only to see Moran standing in front of them with an alien being dressed as a mortician, both ghost-like in appearance and silently chanting words at the team before they attack them.

The Doctor and Clara arrive at the underwater base a few days later, which now appears abandoned. They encounter the apparitions, which lead them to the alien vessel. They examine it and discover the same mysterious symbols carved onto the inside. The ghost-like apparitions then attack them, and in their escape, they find the rest of the team hiding in the base's Faraday cage - where the rest of the crew have been hiding out since the ghosts cannot pass through its walls.

The crew, now led by a deaf woman named Cass, relates, through her translator Lunn, what happened before, but they have no idea what's causing any of it. The Doctor identifies the alien ghost as being from the planet Tivoli, a cowardly species more likely to surrender than attack. The base's automated systems then shifts back into 'day' mode, causing the ghosts to disperse so making it safe for them to leave the chamber. The Doctor and Clara learn that for some reason the ghosts can only appear and operate during the 'night' mode.

Crew of The Drum

In the command centre The Doctor, initially dismissive of the idea of ghosts, becomes excited about the possibilities. Suddenly, the base prematurely switches to 'night' mode, the ghosts having taken over the base's functions so they can operate whenever they wish. Before the crew can restore the ‘day’ cycle Pritchard, the operation's corporate representative who had gone off alone looking for a profit angle, is murdered. He is transformed into another ghost that attempts to attack the others before another crew member is finally able to switch the base back to 'day' mode. Cass opts to have her crew abandon the base and has a rescue submarine called for, only to find that one is already on its way. Much to the crew's annoyance, The Doctor cancels the rescue submarine's arrival, meaning they are now on their own, as he believes the ghosts had made the call for the submarine for a nefarious purpose.

To get answers, The Doctor proposes a plan to capture the ghosts using holographic projections sent by his sonic sunglasses. With the help of the others they manage to lure and trap the ghosts inside the Faraday chamber. Once trapped, Cass finds she can lip-read what the ghosts are chanting - they are repeating the same phrases: ‘the dark, the sword, the forsaken, the temple’. The Doctor recognises these as galactic coordinates and that the ghosts are, in reality, transmitters signalling the co-ordinates to lead some entity to Earth. The signal getting stronger with each new ghost. The co-ordinates ultimately lead to a locked stasis pod located inside a church within the flooded town. The crew retrieve it with a remote submarine, but cannot open it. The Doctor then deduces that the mysterious symbols on the inside of the alien vessel are the co-ordinates in written form. They also re-wire the synapses of anyone who reads them, so that the ghosts can then kill them and make others like themselves to boost the signal.

Ghosts

Without further leads The Doctor concludes that he needs to travel back to when the alien vessel originally crashed in the town, before it was flooded, to find out what is in the pod and the true meaning behind the signal. Just then the ghosts override the base's systems, opening external hatches, so causing the base to begin to flood. The group are forced to run to the TARDIS before they are sealed inside. However, on the way, The Doctor is separated from Clara by the automated isolation doors. Clara must stay in the base with Cass and Lunn as The Doctor can’t use his TARDIS to rescue them; due to the ghosts. While The Doctor takes O'Donnell and Bennett to the TARDIS, Clara and the others wait for them in a dining hall. There Clara, Cass and Lunn witness a new ghost outside the base's window - that of The Doctor.

After leaving the underwater base, in the TARDIS, The Doctor arrives with Bennett and O'Donnell at the Army base in 1980, before it was flooded, on the day the alien vessel landed. They encounter the Tivolian Prentis, still alive at this point, and find that the mysterious symbols have not yet been scratched into the wall on the inside of the alien vessel. Prentis reveals that the alien vessel is actually a hearse carrying a deceased conqueror called The Fisher King.

Back in the future in the underwater base, Clara, Cass and Lunn realise that The Doctor's ghost is uttering a list of their names instead of coordinates. When The Doctor contacts Clara, via her mobile phone, she informs him about his ghost, The Doctor is badly shaken by this certain knowledge of his future. Clara forcefully encourages him to try to change events, but The Doctor argues that he cannot and ultimately accepts the eventuality that he must die to keep events in motion. He tries to get information from his ghost, but instead it unlocks the Faraday cage, releasing the other ghosts. Back in 1980, the Fisher King is revealed to be alive, writing the words on the ship's wall and killing Prentis before fleeing.

An Unhappy TARDIS

On discovering this The Doctor with O'Donnell and Bennett go in search of of the Fisher King but find that they are the ones being hunted. They get separated and O'Donnell is killed by the Fisher King. Bennett chastises The Doctor for allowing O'Donnell to die after The Doctor reveals that the list of names his ghost was repeating was the order in which the crew members will die. Since Clara will be next, The Doctor tells Bennett that he is attempting to save Clara, not himself. He tries to return to the future to achieve this, but the TARDIS won't let him leave - The Doctor is locked in his time stream - and instead goes half an hour back in time. The Doctor and Bennett observe the earlier events, unable to interact or interfere. O'Donnell's ghost appears in the future and steals Clara's phone, her only means of contacting The Doctor. Clara realises that, as Cass refused to allow Lunn into the alien vessel, he never saw the writing on the wall. Therefore, the message is not encoded in his brain, and the ghosts won't attack him. Lunn leaves the cage and locates the phone but the ghosts chase him and manged to trap and lock him inside the main control room.

Leaving Bennett in the TARDIS, The Doctor confronts the Fisher King. The creature reveals that the ghosts he's created will signal his people to send an armada to conquer Earth. It also taunts The Doctor's unwillingness to alter the future, but The Doctor chastises it for violating the souls of those it killed simply for its own ends. The Doctor then tells the Fisher King that he's erased the writing from the spaceship's wall, meaning no-one in the future will discover the message. The Fisher King races back to the ship only to discover the writing still there. He realises The Doctor tricked him and has used one of the power cells to destroy the dam wall, flooding the town and killing the Fisher King. TARDIS Security Protocol 712 activates with Bennett still inside, but The Doctor's whereabouts remain unknown as the town floods.

Due to Lunn failing to return, Clara agrees to accompanying Cass to search for him. After narrowly avoiding being killed by Moran's ghost, Clara and Cass regroup with Lunn in the hangar. As they arrive, the stasis chamber opens and The Doctor climbs out. As the Fisher King is then heard roaring in the distance in the base, the ghosts follow the sound, only to be trapped in the Faraday cage with The Doctor's ghost, revealed to be a hologram The Doctor controlled using his sonic glasses from the stasis chamber.

The Doctor informs the survivors that UNIT will come to cut the Faraday cage from the base with the ghosts inside, and erases the memory of the writing from everyone's mind. After Clara comforts Bennett as he expresses his sorrow over O'Donnell's death, he convinces Lunn to admit his love for Cass. The Doctor and Clara leave in the TARDIS, now fully operational with the ghost threat eliminated. The Doctor tells Clara that the order the people would die in was entirely fictional, but that he places Clara's name second to motivate him into action as soon as possible. Clara questions The Doctor as to how he knew what he had to do, and he informs her that he knew what to make his ghost's hologram say because he had found out through her telling him what it was already saying from the future; a bootstrap paradox.