Le vieil Oliva observe du haut du clocher les habitants de son village de Tchécoslovaquie. Alors qu'un couple de cigognes vole dans le ciel, le directeur de l'école tire et abat l'une d'elles. Il veut l'empailler pour sa collection pédagogique, contre l'avis du maître d'école, Robert, qui préfère qu'on étudie les animaux vivants.
The film starts out with 13-year-old Valerie (Schallerová) sleeping in a gazebo. A thief comes in the night and steals her earrings; when she goes to investigate, she sees a horrific man before he covers his face with a weasel mask. The next day she is swimming in a pool and is then watching the water when the thief's arms suddenly return her earrings to her.
Robert, un employé d’hôtel s'essayant à l'écriture de romans de science-fiction voit un jour ses personnages, une femme et deux enfants venus de la planète Tugador dans la Galaxie Arcana, se matérialiser. Les visiteurs sèment la panique dans le village.
The story involves four teenage comrades who take a rowboat along a "river of time" that flows into a mysterious cave and emerges on the other side onto a strange, primeval landscape. The boy actors were Josef Lukáš (Petr, the main narrator), Petr Herrmann (Toník, who also narrates in part), Zdeněk Husták (Jenda), and Vladimír Bejval (Jirka). As they make their way upstream, they realise that they are travelling progressively farther back in time, and facing various perils as they do so (but learning much about prehistoric life in the process). The animals depicted in Cesta do pravěku were never shown interacting with animals of other periods and it is assumed that different parts of the river represented distinct time periods. The plot is somewhat similar to that of the novel Plutonia (1915) by the Russian palaeontologist Vladimir Obruchev, in which a team of Russian explorers enter the Earth's crust via an Arctic portal (a huge depression in the Earth surface created many millions of years previously by the impact of a giant asteroid, into which prehistoric animals had entered), and follow a river that leads them through a sequence of past geological eras and associated animal life. Some scenes in Cesta do pravěku recall Arthur Conan Doyle's 1912 novel The Lost World, with four male protagonists exploring a prehistoric world where they find evidence of native human habitation, are attacked by a group of enraged pterodactyls, witness a twilight fight between a carnivorous dinosaur and a herbivorous one, encounter a Stegosaurus up-close, and see one of their members (Petr) pursued by a Phorusrhacos.
A group of teenagers are invited to a skiing workshop in the mountains, without being told how or why they were picked as participants. There are eleven of them, but the camp supervisors insist that there should be only ten, and that one of them is an intruder. As the group find themselves cut off from the outside world, strange things begin to happen; the supervisors seem intent to create an atmosphere of hostility, turning the participants against each other, even urging them to kill each other. The supervisors eventually reveal themselves as extraterrestrials who demand that the group pick one among them to be sacrificed. They refuse, however, and in a panic set the cottage on fire and make a narrow escape on a lift used for timber transport, leaving no man behind.