In the 26th century, the planet Heaven, has been designated as a cemetery for the dead of various races, of which there are many at the moment, as Earth and Draconia are in the middle of a war with the Daleks. In addition to the military presence and various civilian settlements on the planet, there is also a community of Travelers and a group of archeologists, led by Prof. Bernice Summerfield. Bernice (Benny to her friends), is looking for clues to the function a mysterious arch-shaped monument, left by Heaven's long extinct original inhabitants about whom nothing is known. In the main settlement of Joycetown, there is also a religious sect called the Church of the Vacuum, led by brother Phaedrus, who preach that the universe is without meaning and that people should give themselves over to the vacuum of space. In the middle of one of their rituals where a member of the group is being sacrificed the victim is taken over by an alien presence, that announces it is coming to Heaven and the Church must do its bidding. The Doctor and Ace arrive on the planet looking for a book the Doctor wants called the 'Papers of Felsecar', which he believes he can find in a library of forbidden texts the Earth authorities have stored on Heaven. However, the librarian claims he hasn't seen the book. Outside Ace bumps into one of the travelers called Jan, whom the Doctor promptly saves from the attentions of Kale, one of the soldiers station on Heaven. Ace goes with Jan back to the camp and meets his friends, including Roisa, Jan's on again off again lover, Maire (a Dalek killer who wears one of their guns as a trophy) and Benny. Walking back she tells Benny about the TARDIS and her own background. As one of Benny's specialist subjects is 20th-century history, she knows that no matter how strange her story, Ace is telling the truth. Also at the camp is Christopher, the traveler's sexless high priest, who is beginning to sense evil approaching.