An accident occurs in a laboratory conducting experiments on animals. Some animals manage to escape. And after a while, an epidemic breaks out... A strange virus spreads at an alarming speed, first affecting animals, and then people. Men and women, one after another, begin to lose their human appearance and acquire signs of their ancient ancestors. Paleontologist Anna Monier, together with other scientists, is trying to find the original source of the virus. The UN Security Council decides to isolate the infected, called "erectuses". So, in case of danger, it will be easier to destroy them... This seems humane. But the virus affects Anna’s lover, the future father of her child. Can't they be saved? And her beloved will turn into a primitive creature? But what if there is more humanity in those who are called “homo erectus” than in those who proudly call themselves “humans” - “homo sapiens”...