An organism crash lands in Antarctica, and parts of its biomass are destroyed. The organism used to be an explorer, an ambassador, and a missionary that traveled across the cosmos. It tries to build an escape boat, but human beings discover the mysterious organism and try to destroy it. In order to survive, it disintegrates and has its disparate cells latch on to the bodies of a group of nearby Norwegian men. It can occupy their bodies and eavesdrop on their minds, although the men remain in control of their minds and movements. The organism finds the human creatures quite puzzling because they are not shapeshifters. Two of the men, Macready and Copper, find another man dead and burnt. The organism had been inside him and the men perform an autopsy. The organism discovers that men have brains, and feels threatened by this organ that it calls a cancerous tumor fighting for control. The organism says that it is used to incorporating souls, not rooming with them. The people seem to sense that there is a malignant presence among them and Macready invents a detector to find the organism. The humans turn on each other in order to fight it off. But the organism jumps from host to host, staying alive. It decides that it must continue to occupy their bodies in order to save them from themselves.