Rassmussen

Professor Gagan Rassmussen, the lead researcher on the Le Verrier Space Station in orbit around Neptune sometime in the 38th century, addresses a camera to record a message. Rassmussen warns the viewer not to watch this but explaining how the video, assembled from various recordings made over the last few hours, will help put together events leading to this point. Rassmussen continues to narrate through the story as events are played out.

Hours earlier, a rescue ship from Triton arrives at the Le Verrier Space Station in response to a sudden absence of communications with the station, with four soldiers: Nagata, Chopra, Deep-Ando, and 474, a bio-enginered grunt with low intelligence though cares for Chopra. They find the space station empty with no signs of its crew.

They soon encounter The Doctor and Clara who have just arrived themselves. The Doctor uses his psychic paper to pass them off as Engineering Stress Assessors. After introductions the group find themselves being chased by creatures made of a congealed sand-like substance (later called ‘Sandmen’ by Clara) that are not harmed by any of their weapons. Deep-Ando though finds himself separated from the others as they take shelter. As the others try to contact him, Clara is inadvertently dragged into a coffin-like pod.

Engineering Stress Assessors

Nagata though identifies the pod as a harmless Morpheus sleep pod, which is claimed to compress a whole month worth of sleep into a five minute period, allowing people to work, like herself, around the clock, though Chopra insists that the technology is unnatural and has refused to use it himself.

After freeing Clara they find, hiding in another pod, Morpheus' inventor, Professor Gagan Rassmussen who explains that it works by sending out an electronic signal to the brain, changing it to enable this process. He reveals that they were testing the next generation of the Morpheus devices when the Sandmen first appeared. The Doctor fears that they are a side effect, that the Sandmen creatures are made up of the dust that collects in the eye and, after consuming the host and other crew on the space station, are now after them. They find that the song associated with Morpheus, ‘Mr. Sandman’, lures the creatures to them. Meanwhile, Deep-Ando is killed while trying to escape the Sandman.

Chopra

The Doctor, Clara, Rassmussen and the remaining members of the rescue team are unable to come to Deep-Ando's aid as they have problems of their own when the Sandman attack them at the same time as the space station's gravity shields fail, threatening to drag the space station into Neptune. Though the increased gravity causes the Sandmen to start to disintegrate, they are able to kill Rassmussen in the chaos.

The Doctor manages to restore the gravity shields, and before the Sandmen can react, The Doctor, Clara, and Nagata take shelter in a kitchen freezer while Chopra and 474 attempt to make it back to the rescue ship. 474 sacrifices herself to carry Chopra through a wall of fire created by the gravity shield failure, while Chopra eventually makes it back but is also killed by a Sandman waiting in the rescue ship.

In the freezer, The Doctor observes the Sandmen appear to be blind, and the three of them escape. Finding a moment's respite, The Doctor discovers through his sonic sunglasses that a variety of video signals have been transmitted, and shows these to Clara and Nagata, all depicting views from their points of view, except that there are no cameras to have taken these, nor any from Chopra's point-of-view. Instead, this is a result of being subjected to Morpheus, that it infects the user and consumes them from the inside. The Sandmen are able to use these signals, generated by those subjected to Morpheus, to hunt down their prey despite their blindness. The Doctor promises he can reverse the process in Clara and Nagata once they are back to the TARDIS.

Nagata

Despite apparently being killed by a Sandman, when they return to the rescue ship, they discover Rassmussen there, along with a Morpheus pod he claims contains the first patient of the Morpheus process from five years prior. Rassmussen admits that his goal is to aid the Sandmen to leave the space station and get to Triton, from which they can infect the rest of the solar system. He also reveals that the failure of the gravity shield was planned to allow him to send the pod to the rescue ship without attracting attention.

Rassmussen then attempts to lock The Doctor, Clara and Nagata in a part of the rescue ship with the patient, now a Sandman, but The Doctor engineers their escape, and Nagata shoots Rassmussen before he can launch the rescue ship. The Doctor leads them all to the TARDIS, realising that the events that have happened are too choreographed to seem like a real danger. As they are surrounded by Sandmen, The Doctor deactivates the gravity shields, causing the Sandmen to disintegrate, and the three escape into the TARDIS as the station plummets into Neptune.

As the TARDIS departs, Rassmussen reveals in his narration that he had been a Sandman all this time and that all the events of the past few hours were engineered to tell a story that would keep the viewer enticed to continue to watch the recordings in order to transmit the Morpheus signal (disguised as glitches in the video recording) to them, thus assuring that the Sandman would spread to anyone that watched it, hence his duplicitous warning at the start. Rassmussen then disintegrates into sand as the transmission ends.