An adventurous story about traveling through time and friendship across history. One summer morning, siblings Klára and David find a frightened little boy in front of their parents' cottage. They would like to leave the care of the strangely behaving child to someone more mature, but they are alone, so they have no choice but to take little Artur under their wing and try to return him to the place from which he wandered off. That same morning, another unusual visitor arrives at Dr. Hávová's veterinary clinic. A man whose behavior is somewhat unusual for today's world. He calls himself Robert and tries to pay for the food he has been given with gold coins. Independently of each other, the protagonists of the story uncover clues that lead to the cottage of a local eccentric named Kobza and further to a cave called Devil's Mouth. Is this cave really a gateway to another world and time? And will Artur and Robert manage to get back home through it?