The TARDIS picks up a strange transmission and materializes on Acheron Base, a research facility on Pluto, in the year 2103. As the Doctor and Peri begin to investigate they run into Fiske, one of the base crew, who demands to know who they are. Fiske is naturally suspicious since he and his associate, Blake, have just returned from an expedition to Pluto's moon, Charon, to find the base completely deserted. The Doctor soon discovers that the key to the disappearance of the base personnel may have something to do with a mysterious artifact that Blake found on Charon. However, Blake has become unstable and has made it impossible for any of them to summon help. Peri starts seeing ghosts, and the commander of the base turns up from nowhere. It soon becomes apparent that not everyone on the base is who he appears; yet the Doctor must work with them if he hopes to uncover the truth. He soon begins to realize that they are facing a terrifying force that is as old as Time itself.