Ola loved her holidays. Especially those spent with Bartek and Natan by the sea, in a small fishing village, where everything smelled of salt and sea grass. They ran to the beach together every morning, racing between the boats, making sandcastles and attaching their ears to the sound of the waves, looking for the stories hidden in it. One cloudy afternoon, as the wind rocked the umbrellas and the seagulls screamed overhead, Bartek tripped over something hard sticking out of the wet sand. He bent down, pushed the sand away with his hand and... found a rusty metal can. He called out to Ola and Natan. They opened it together. Inside was an old map. It was faded, but you could recognise an island in the middle of the lake, strange drawings of palm trees and... something that looked like a very old compass with a split pointer. - This must be a treasure map! - whispered Ola with a baked face. - Or some kind of clue to something fascinating - added Natan. Bartek was already running back to the cottage to get his rucksack, because he knew they couldn't wait a moment longer. On the back of the map someone had written in pencil: "Dare to set foot on the Shark's Tooth at dawn. Only then will you find what Captain Red Beard has lost." Bartek remembered that the Shark's Tooth is a rock at the other end of the island that can only be reached by boat. Infected with enthusiasm, all three set off for the marina. They took oars, torches, a bottle of water and, of course, a map. For a moment Ola hesitated, as she didn't like going by boat, but Bartek's smile and the hand extended by Natan convinced her. Already rowing across the calm surface of the lake, they noticed that darkness was gathering faster than they had expected. Suddenly, the boat glided over something that shimmered just under the water. It was a shiny, golden fragment of something large and heavy. As Ola busied herself watching the map, Bartek leaned too far over the side and Just then, as a quiet splashing broke through the sounds of the evening, they felt the boat begin to sink lower, as if some invisible force was pulling it deeper. Natan shouted: - What is happening! Strange, shimmering bubbles appeared around them and silvery fish began to dance in a circle. The boat, the map and the three friends slowly sank into the thick fog floating over the surface of the lake....