Geologist Semyon Vasiliev found himself in the Stone Age not on a whim. Having started a new life from scratch - by collecting shells on a river shallow - he managed not only to escape, but also to find worthy use of his abilities in the virgin world of Mammoth. However, an environmental disaster soon began. The climate changed sharply, hunger and civil strife threatened to destroy both people and animals. When the first bloody battles were in the past, Semyon found himself in the role of the informal leader of several Cro-Magnon tribes, a crowd of Neanderthals and a group of relict hominids-pithecanthropes. This fragile unity will face the most difficult tests: famine, invasion of strangers, betrayal of allies. In addition, people of different species do not like, do not understand each other and are ready at any moment to start a war of extermination...