Black Threads, or The Mystery of the Underground Passage

It's adults who think that a trip to the sea is the height of bliss. But what if you're going to a boring sanatorium near Sochi with your grandmother, like Kira? Or stuck in your room due to a fight with your father, like Matvey? But everything changes when the children meet a huge shaggy dog and a mysterious girl in an ancient dress: a door to another world, dragons, luxurious fruit gardens, and fierce warriors from the last century will make this vacation unforgettable! But will an ordinary boy and girl be able to unravel the magical black threads and return home?