The astronauts of the 2nd expedition to Mars are mistaken for mental patients and sent to an asylum. To prove they are mere psychic manifestations, a physician tries to "cure" them by killing them.
The fourth expedition finds all the Martians have evidently died due to chicken pox exposure. The crew's historian finds himself empathizing with the vanished civilization and decides to kill the rest of the crew in the hopes of preventing Mars' colonization.
Tomas encounters a Martian but they seem to exist in different time frames; neither one can agree which of them is from the past and which from the present.